Actually, in general I do like the idea... After I read a post that's been posted a certain time ago, I then can assume that it won't change again. This closes a security loop hole, in my opinion, since a moderator can then rest assured that old posts won't be changed "behind his/her back", especially on forums that don't require moderator approval for posts. I noticed that if I edit any of my posts, it is usually in a matter of minutes. Also, if the limit has passed, one can always add an "update" post to the thread, e.g. for your problem with updating hacks.
