Inspired by events in this question: Why didn't the Ex-Wife work?
The purpose of a comment in Stack Exchange as I understand is to improve a question as given here:
Comments are for, and should be used 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).
When the first one is intended, the author of the question should add relevant details in the question itself, instead of replying in the comment because that may be a valid point and should include the required details in the question.
Now, after that step, what is the necessity of the comment still staying there?
Should people use "no longer needed"? Should those comments be removed? After all, the comment has served its purpose.