I must say, it is incredibly frustrating when you comment on a submission, and then you realize you have something else to add. It will get pretty annoying for anyone reading the comments to see the same person commenting over and over again in order to add more. You could copy your post, delete it, and make a new one, but if someone commented afterward, it would look very confusing to anyone reading, because it would look out of order chronologically and it would look like the person told the future.
What I am proposing is that we should be able to edit our comments. This could work the same way as editing posts on the forums (iirc, we can only edit within 10 minutes of posting). It would be so much more convenient for both the commenter and the reader. Please take this into consideration.
Edit: I misspelled editing in the thread title. :|