Many comments highlight a problem with an answer or a question. The author often acts on the comments and addresses them, making them obsolete.
It would be nice to let authors explicitly address comments as both helpful and "to be obsoleted" when updating answers/questions. (This is now typically done with yet more comments, which somehow pollute the final answer). In this sense, comments can often be seen as issue-tickets in a bug-tracking system. Once resolved, they don't add much to the discussion.
Another take on this is to provide actual context for the comment - if a comment was made while viewing version 1 of an answer, after version 3 is reached, the comment should contain a small (v1) annotation with a link to the version it was written for...