Why is there such a strange regulation? I understand that it wouldn't be a good idea to allow editing comments if they have already been replied to, but what they haven't? Sometimes I come back to my comments that I left two or three days ago and upon re-reading them I get some enlightenment on how to phrase my thoughts in those comments better - this would only add to the value of my comments, but adding another comments to my previous comments just for the sake of editing makes the page kind of overloaded with redundancy.
Comments are, by design, second-class citizens. Since StackExchange is a Q&A engine and not a discussion forum, the focus is strictly on the quality of answers. Comments are kind of expected to be quick-and-dirty pointers whose main purpose is, again, to help improve the answers. In fact, originally the engine didn't allow comments to be edited at all; the five-minute grace period was built in at a later date.
However, it is not entirely impossible to improve a comment of yours after the five minutes have passed. If your comment is the last one in the thread or hasn't been replied to yet, the consensus on the meta Meta is that you should simply delete the comment and re-submit it with the improved wording/spelling/punctuation. Of course, you will lose any upvotes the original comment might have received in the mean time, but that's the price.
Note that moderators can edit anybody's comments at any time, so they might fix particularly egregious typos from time to time, at least theoretically, as a courtesy. (Nohat once fixed a comment of mine in which I had totally messed up the link formatting.) However, generally speaking, moderators have much more important things to do, so you can't expect them to proof-read any or all comments; that's simply not part of their job description.