Originally, comments addressed a legitimate need to add short annotations to a post, to note a caveat or mistake, or ask for clarification. My understanding was the comment would eventually lead to improvements in the post. The author of the post (or in some cases an editor) would read the comment, improve the post to address the comment's concern, and then delete the comment.
I don't like the feature creep I've seen since comments were introduced. They got longer, everyone but the commenter lost the ability to delete, they got votes, and now there's even a badge for votes on comments.
I don't see how any of that, and especially editing comments, contributes to the ultimate goal of the sites: better questions and answers.