I find time limit on editing comments annoying and doubt that it is needed at all. My problem is typos that I can't fix if I noticed them too late. If my comment is not answered I post the corrected version and then delete the old version. But if my comment is already answered, I can't do even this, and should see the ugly typo every time I visit the page.
Well, I can understand the answer like: removing time restriction requires too big changes in software and we will not do it, but I don't like false excuses.