I often find myself posting a comment simply to point out a code mistake in my own old comment, or to add a caveat to another user's comment. To avoid polluting SO/SE with lots of redundant comments, it would be nice to have a privilege to enable editing of all comments, old and new, our own and others', to enable basic curating of this content.
Flagging existing comments is not at all the same. For example, in case of a typo, the goal should be to give credit to the original commenter for providing useful input, while correcting a minor aspect of that input.
The main goal of this feature would be to avoid several comments which could have been merged into a single, much more useful and concise comment. Comments take up valuable screen space, so they should be as flexible as answers.