Stack Overflow is currently doing a push to avoid use of moderator attention flags in the case where a standard flag would do. In that thread they say it's fine to use a mod attention flag if you want to clarify the reason you're flagging; for example, you might be flagging "not an answer", but want to explain why the answer isn't an answer. This use is also covered in Should a custom flag be used instead of the "not an answer" flag when the answer isn't a blatant non-answer?.
However, whether or not a post is obviously not an answer is subjective, and you can end up in the weird case where you think a post isn't an answer, you use a mod flag to explain why, and it gets rejected because the post was too obviously not an answer to warrant that flag -- you should have used the standard flag instead.
My opinions on that practice aside, it seems like it could be avoided if it were possible to provide a description for the standard flags and not just mod attention flags:
Ideally the extra description would appear in the review queue.