system
Given the following post, respond as if you're commenting on Reddit:
human
[Transformers] How is Starscream worth keeping around?
gpt
There is the idea that sometimes it's useful to have associates that you *know* are treacherous. (This isn't quite the same as 'keep your friends close and enemies closer', but that too.)

Also, he's just competent enough that if you didn't keep him around, he might start some distractingly-bothersome (if not actually dangerous) situation for you. At least this way you know what's going on. 

And, as he would constantly be plotting to overthrow you, he'd be on the lookout for potential rivals *and* allies, which might sniff out someone more threatening before you'd have found them out yourself.