Sometimes a comment is posted that answers a question. This is not even always wrong--sometimes the comment is insufficiently detailed to constitute a good answer.
Then, sometime later, a good, simple, thorough answer that includes all the information in the comment is posted.
At this point, is it appropriate for the comment to be flagged as obsolete?
Sometimes, keeping the comment would serve to highlight an idea important in the question or answer. But often, it seems to me that a good comment can be obsoleted by an answer.