title: Why is READ_COMMITTED_SNAPSHOT not on by default?
body: <p>Simple question?</p>
  
  <p><strong>Why is <code>READ_COMMITTED_SNAPSHOT</code> not on by default?</strong></p>
  
  <p>I'm guessing either backwards compatibility, performance, or both?</p>
  
  <p><strong>[Edit]</strong> Note that I'm interested in the effect relating to the READ_COMMITTED isolation level, and not the snapshot isolation level.</p>
  
  <p>Why would this be a breaking-change, as it holds less locks, and still doesn't read non-committed rows?</p>