Perhaps I'm not checking HN regularly that day and make an offhand comment. In that case, if I come back a few hours later and recognize a boo-boo, it would be nice to be able to correct my mistake to avoid a permanent record of my foolishness.
How can you make a mistake in a comment? If you don't mean what you write don't post it... And don't worry about people calling you foolish (I am sure there is a famous quote for that to put it more eloquently).
Also, note that there is no post preview when you submit so being able to modify after you post fills that functionality gap.
Perhaps a reasonable compromise would be to have the ability to add a timestamped addendum, without being able to edit the original. I would still want the one-hour free edit though.