Hmm... I'm also not in favor of this. I guess I just see the ability to edit/delete information from the public ticket thread as a feature that could easily be abused and then, that aside, you're still left with the issue of notifying customers and staff that those changes have been made.
More important than that, though, from my perspective, it's a ticket system, not a forum. Tickets aren't intended to be discussions. They're a procedural step-through of requests/responses/actions that need to be taken in order to solve a problem. Being able to edit or delete an earlier entry in the ticket history defeats that purpose. It's like hitting step 12 in a set of instructions and finding that it says "OK, now go back and read step 1 again and start over, because we changed it while you were doing step 7" and suddenly you end up building an entertainment center instead of the desk you thought you purchased.
So even if there are errors and corrections in earlier entries that need to be dealt with, I think it's important that they are dealt with in the course of the ticket history. "Oh, I made a mistake back there." needs to be said when it's found so it can be dealt with at that time, according to the steps that have already been taken, not just corrected in the original entry.