user11841: Sometimes the comments below a question partially answer the question. I got a message from a moderator stating to refrain from answering in comments.
Answering a question in the comment may be due to multiple reasons:

No time to give a full answer
To give a clue to the OP as to what direction the person should do research to find the answer. For example, the comment may contain some terms which the OP might not have been familiar with, which if researched would give a full answer.
When the question might seem too basic for the person who answers that he/she considers it not worth to take the time to find the references or to expand the comments to give a full answer

There many be many more reasons. This behavior of answering or partially answering in the comments is common in Stackoverflow.
Is there a policy regarding this in biology.stackexchange.com?
What are the reasons to withhold answering in the comments? When it might be considered appropriate?

user43353: See here for a discussion on this from way back.

What are the reasons to withhold answering in the comments?

You already posed a few scenarios. They are all valid in my opinion. Some questions are misguided and have simple yes or no answers, or 'not applicable' answers. I think most answers of this kind should occur in the comments, because the question and its answer would not be suitable for archiving. These won't come handy for future users; I think the SE should be an ever growing repository of good questions with good answers (that can be referred to by future users), not shoddy questions with cheap answers. Not that cheap answers are necessarily incorrect!
Anything which is solid enough to stand as a good answer for a good question should be indexed, and comments aren't indexed. That's the gist of it.

When it might be considered appropriate?

I think comments that partially answer the question are okay. These comments tend to be informative, or challenge the question. Often in biology, you can tackle a question from multiple angles and with different scopes. There must be room for challenging the question. Comments typically highlight this well, and I haven't seen many examples of answering in the comments so far, and I check in daily. I also think that if you can help someone, but don't have time to write a beautiful, insightful, well-referenced and well-formatted answer, you should be able to pitch in. This is all voluntary, after all.
tl;dr i think partial answers in comment sections shouldn't be discouraged

user27148: disclaimer: this is just my own opinion as a user and, though it may be reflected in how I currently think about comment flags, this is my own personal answer and not representative of the other mods
Some reasons to discourage partial answers in comments:

Comments cannot be downvoted. Therefore, incorrect comment answers can only spark comment arguments. Especially when a question reaches the HNQ, a comment answer might attract a lot of upvotes even if it's wrong just because it sounds correct or pithy to the masses.
Comments cannot be accepted; if a comment answers the OP's question, OP may leave and not return and then cannot accept a future good answer.
Answers (often) require references; answering in a comment to bypass giving references bypasses all the reasons that we have chosen to require references.
Answers in comments may discourage actual answers. Although it's been encouraged on the main meta to answer questions even if it means effectively copying a comment into an answer, people feel uncomfortable doing that, and so a question remains without a true answer for future visitors to the site.

I am far more tolerant (and sometimes may be seen as "comment answering" myself) when the answer falls into one of these categories:

There is a major flaw or oversight in the question that makes it not really answerable. The comment answer actually addresses this flaw, rather than answering the question.
There is a trivial way to answer the question in an accessible resource, or with search terms OP hasn't considered.
Comments that provide very generic answers like "you should discuss this with your physician" while closing medical advice questions, or "discuss with your instructor" for homework help. I don't really see these as answers, personally, but I think as long as they are vague and not adding a diagnosis ("Sounds like you have diabetes, go talk to your doctor!") I think they are fine.

For (1) and (2) here, I feel like although these sorts of comments are answer-like, they serve to provide feedback on the question because if the OP were to follow up with those corrections or research suggestions they could improve the question to address their remaining uncertainty.

Summary
Therefore, my suggestion would be that comments in the first set of categories be removed, while comments in the second category remain.

user9943: I tend to say to users that have hidden answers in the comments that 

comments are for comments, answers are for answers

It's a cliche, but it's just what it is. The help center is also quite explicit about (text shortened and markups are mine):

When should I comment?
  You should submit a comment if you want to:

Request clarification from the author; 
Leave constructive criticism that guides the author in improving the post; 
Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the
  question has been updated).

When shouldn't I comment? Comments are not recommended for:

Answering a question or providing an alternate solution to an existing
  answer; instead, post an actual answer (or edit to expand an existing
  one);


The bottom line is that comments are transient and can, and often are, deleted by the mods or OP at some point. Leave comments for clarifications, tips and critiques. Leave answers for answers. 
For instance, this month (27 days and counting), 135 comments have been deleted by the mod team alone. These deleted comments are invisible to the regular user, cannot be upvoted any longer, any upvotes on them evaporate. Leaving answers allows users to downvote (comments cannot be) and allow them to be accepted by OP. This model is what the SE network is built upon.       

user22057: Is there a policy regarding this in biology.stackexchange.com?
Yes. The policy is quite clear. It is the general policy of Stack Exchange that appears in the question comment box of every Stack Exchange site:

Use comments to ask for more information or suggest improvements. Avoid answering questions in comments

Although individual Stack Exchange sites can make their own policy on certain things like what questions are on topic, or how many votes are needed to close a question, this is not one of them. If it were, there would be sites that did not have this message in the comment box.
Understanding the policy
This policy exists because breaking it would break the whole model underpinning Stack Exchange. It is worth re-reading the Tour where this is set out after the centred italicized statement:

We're a little bit different from other sites. Here's how:

The Stack Exchange model (voting, editing etc.) is devised to provide a set of high-quality answers to high-quality questions. It is not an answering service with dialogue between individuals whose knowledge and ability is taken on trust. As the Tour goes on to say:

This site is all about getting answers. It's not a discussion forum. There's no chit-chat.

And that is why each site has an extensive Help on writing good answers (not half answers).
For those who have the time it may be worthwhile watching a Google Tech Talk given by one of the founders of the original Stack Exchange — Stack Overflow — in which the model is expounded in more detail.
How the busy can help on SE Biology
I understand the poster’s desire to help people with questions when he is pressed for time. However breaking the Stack Exchange model is not the answer. Possibilities are:

Wait and answer later when you have time if the question has not been answered. If it has, you are saved the bother (or can comment on an answer), if it has not, then if the poster really wants to know he will be grateful. If he is in a hurry, too bad. Other people’s deadlines are not our concern.
Don’t answer. Is it really such a good question? As it says in the Help, [“Not all questions can or should be answered here.”] (https://biology.stackexchange.com/help/how-to-answer).
If appropriate, point the poster to a suitable article that will help him to find the answer himself, suggesting he comes back with specific problems which you will look at when you find time. Often people don’t know how to search effectively.

Postscript
I have not commented on the argument (if it is meant to be an argument): “This behavior of answering or partially answering in the comments is common in Stackoverflow”. My attitude to such behaviour should be apparent. Fortunately it is not common on SE Biology. My detailed arguments against the practice can be found in answer I gave to a question on this topic on SE English Language and Usage Meta. 
Oh, and I would have answered earlier, but I was busy. I trust my answer was worth waiting for.

11841: <p>Sometimes the comments below a question partially answer the question. I got a message from a moderator stating to refrain from answering in comments.</p>

<p>Answering a question in the comment may be due to multiple reasons:</p>

<ol>
<li>No time to give a full answer</li>
<li>To give a clue to the OP as to what direction the person should do research to find the answer. For example, the comment may contain some terms which the OP might not have been familiar with, which if researched would give a full answer.</li>
<li>When the question might seem too basic for the person who answers that he/she considers it not worth to take the time to find the references or to expand the comments to give a full answer</li>
</ol>

<p>There many be many more reasons. This behavior of answering or partially answering in the comments is common in Stackoverflow.</p>

<p>Is there a policy regarding this in biology.stackexchange.com?</p>

<p>What are the reasons to withhold answering in the comments? When it might be considered appropriate?</p>
 43353: <p>See <a href="https://biology.meta.stackexchange.com/questions/552/comments-with-answers">here</a> for a discussion on this from way back.</p>

<blockquote>
  <p>What are the reasons to withhold answering in the comments?</p>
</blockquote>

<p>You already posed a few scenarios. They are all valid in my opinion. Some questions are misguided and have simple yes or no answers, or 'not applicable' answers. I think most answers of this kind should occur in the comments, because the question and its answer would not be suitable for archiving. These won't come handy for future users; I think the SE should be an ever growing repository of good questions with good answers (that can be referred to by future users), not shoddy questions with cheap answers. Not that cheap answers are necessarily incorrect!</p>

<p>Anything which is solid enough to stand as a good answer for a good question should be indexed, and comments aren't indexed. That's the gist of it.</p>

<blockquote>
  <p>When it might be considered appropriate?</p>
</blockquote>

<p>I think comments that partially answer the question are okay. These comments tend to be informative, or challenge the question. Often in biology, you can tackle a question from multiple angles and with different scopes. There must be room for challenging the question. Comments typically highlight this well, and I haven't seen many examples of answering in the comments so far, and I check in daily. I also think that if you can help someone, but don't have time to write a beautiful, insightful, well-referenced and well-formatted answer, you should be able to pitch in. <em>This is all voluntary, after all.</em></p>

<p><strong>tl;dr i think partial answers in comment sections shouldn't be discouraged</strong></p>
 27148: <p><em>disclaimer: this is just my own opinion as a user and, though it may be reflected in how I currently think about comment flags, this is my own personal answer and not representative of the other mods</em></p>

<h2>Some reasons to discourage partial answers in comments:</h2>

<ol>
<li>Comments cannot be downvoted. Therefore, incorrect comment answers can only spark comment arguments. Especially when a question reaches the HNQ, a comment answer might attract a lot of upvotes even if it's wrong just because it sounds correct or pithy to the masses.</li>
<li>Comments cannot be accepted; if a comment answers the OP's question, OP may leave and not return and then cannot accept a future good answer.</li>
<li>Answers (often) require references; answering in a comment to bypass giving references bypasses all the reasons that we have chosen to require references.</li>
<li>Answers in comments may discourage actual answers. Although it's been encouraged on the main meta to answer questions even if it means effectively copying a comment into an answer, people feel uncomfortable doing that, and so a question remains without a true answer for future visitors to the site.</li>
</ol>

<h2>I am far more tolerant (and sometimes may be seen as "comment answering" myself) when the answer falls into one of these categories:</h2>

<ol>
<li>There is a major flaw or oversight in the question that makes it not really answerable. The comment answer actually addresses this flaw, rather than answering the question.</li>
<li>There is a trivial way to answer the question in an accessible resource, or with search terms OP hasn't considered.</li>
<li>Comments that provide very generic answers like "you should discuss this with your physician" while closing medical advice questions, or "discuss with your instructor" for homework help. I don't really see these as answers, personally, but I think as long as they are vague and not adding a diagnosis ("Sounds like you have diabetes, go talk to your doctor!") I think they are fine.</li>
</ol>

<p>For (1) and (2) here, I feel like although these sorts of comments are answer-like, they serve to provide feedback on the question because if the OP were to follow up with those corrections or research suggestions they could improve the question to address their remaining uncertainty.</p>

<hr>

<h2>Summary</h2>

<p>Therefore, my suggestion would be that comments in the first set of categories be removed, while comments in the second category remain.</p>
 9943: <p>I tend to say to users that have hidden answers in the comments that </p>

<blockquote>
  <p>comments are for comments, answers are for answers</p>
</blockquote>

<p>It's a cliche, but it's just what it is. The <a href="https://biology.stackexchange.com/help/privileges/comment">help center</a> is also quite explicit about (text shortened and markups are mine):</p>

<blockquote>
  <p>When should I comment?
  You should submit a comment if you want to:</p>
  
  <ul>
  <li>Request <strong>clarification</strong> from the author; </li>
  <li>Leave constructive criticism that <strong>guides the author in improving the post</strong>; </li>
  <li>Add relevant but <strong>minor or transient information</strong> to a post (<em>e.g.</em> a link to a related question, or an alert to the author that the
  question has been updated).</li>
  </ul>
  
  <p>When shouldn't I comment? Comments are not recommended for:</p>
  
  <ul>
  <li><strong>Answering a question</strong> or providing an alternate solution to an existing
  answer; instead, <strong>post an actual answer</strong> (or edit to expand an existing
  one);</li>
  </ul>
</blockquote>

<p>The bottom line is that comments are transient and can, and often are, deleted by the mods or OP at some point. Leave comments for clarifications, tips and critiques. Leave answers for answers. </p>

<p>For instance, this month (27 days and counting), 135 comments have been deleted by the mod team alone. These deleted comments are invisible to the regular user, cannot be upvoted any longer, any upvotes on them evaporate. Leaving answers allows users to downvote (comments cannot be) and allow them to be accepted by OP. This model is what the SE network is built upon.       </p>
 22057: <p><strong>Is there a policy regarding this in biology.stackexchange.com?</strong></p>

<p>Yes. The policy is quite clear. It is the general policy of Stack Exchange that appears in the question comment box of <em>every</em> Stack Exchange site:</p>

<blockquote>
  <p>Use comments to ask for more information or suggest improvements. Avoid answering questions in comments</p>
</blockquote>

<p>Although individual Stack Exchange sites can make their own policy on certain things like what questions are on topic, or how many votes are needed to close a question, this is <strong>not</strong> one of them. If it were, there would be sites that did not have this message in the comment box.</p>

<p><strong>Understanding the policy</strong></p>

<p>This policy exists because breaking it would break the whole model underpinning Stack Exchange. It is worth re-reading the <a href="https://biology.stackexchange.com/tour">Tour</a> where this is set out after the centred italicized statement:</p>

<blockquote>
  <p>We're a little bit different from other sites. Here's how:</p>
</blockquote>

<p>The Stack Exchange model (voting, editing etc.) is devised to provide a set of high-quality answers to high-quality questions. It is not an answering service with dialogue between individuals whose knowledge and ability is taken on trust. As the Tour goes on to say:</p>

<blockquote>
  <p>This site is all about getting answers. It's not a discussion forum. There's no chit-chat.</p>
</blockquote>

<p>And that is why each site has an extensive Help on writing good answers (not half answers).</p>

<p>For those who have the time it may be worthwhile watching a <a href="https://www.youtube.com/watch?v=NWHfY_lvKIQ" rel="nofollow noreferrer">Google Tech Talk</a> given by one of the founders of the original Stack Exchange — Stack Overflow — in which the model is expounded in more detail.</p>

<p><strong>How the busy can help on SE Biology</strong></p>

<p>I understand the poster’s desire to help people with questions when he is pressed for time. However breaking the Stack Exchange model is not the answer. Possibilities are:</p>

<ol>
<li>Wait and answer later when you have time if the question has not been answered. If it has, you are saved the bother (or can comment on an answer), if it has not, then if the poster really wants to know he will be grateful. If he is in a hurry, too bad. Other people’s deadlines are not our concern.</li>
<li>Don’t answer. Is it really such a good question? As it says in the Help, [“Not all questions can or should be answered here.”] (<a href="https://biology.stackexchange.com/help/how-to-answer">https://biology.stackexchange.com/help/how-to-answer</a>).</li>
<li>If appropriate, point the poster to a suitable article that will help him to find the answer himself, suggesting he comes back with specific problems which you will look at when you find time. Often people don’t know how to search effectively.</li>
</ol>

<p><strong>Postscript</strong></p>

<p>I have not commented on the argument (if it is meant to be an argument): “This behavior of answering or partially answering in the comments is common in Stackoverflow”. My attitude to such behaviour should be apparent. Fortunately it is not common on SE Biology. My detailed arguments against the practice can be found in answer I gave to a question on this topic on <a href="https://biology.meta.stackexchange.com/questions/4044/are-comments-that-partially-answer-the-question-discouraged">SE English Language and Usage Meta</a>. </p>

<p>Oh, and I would have answered earlier, but I was busy. I trust my answer was worth waiting for.</p>
