Can you provide a scenario where it'd be useful? I can't imagine wanting to stop a post from being edited... unless it was to preserve something, or to prevent someone from editing their post before a moderator issues action against them (but that's what quoting posts is for).
Well, I moderate at a forum called 'AMXBans.de', it's a half life ban system with a web interface. (I'm not trying to get people there, else you won't know what it's about)
I am in charge of Translations there. When a translator posted a thread with all the lines, I closed it, I did this with all posts he made, to be sure I had backups of all versions, and to be sure he didn't just update the thread without me knowing it.
That's something it's useful for, I'm sure there are more examples.
Well if you're offering users the option to edit their post, then stopping them editing it to update it (new content maybe?), it's simply revoking that feature IMO.
I do like the idea of it, don't get me wrong, it's just something I'm trying to imagine in my own personal use