This is the biggest reason for keeping edit time short.
The way that vBulletin functions on my end, if a user deletes a post during the edit time, I can pull up the original post; if a user changes the content of the post, there is no way to retrieve the original.
So, a troll comes in, posts something inflammatory, then changes the content of their post after a couple of replies (but before someone reports it) the person(s) who replied look like they were trolling instead of the real bad guy. Believe it or not, people actually do this sort of thing.
Myself, as an Admin here, have the ability to edit my posts at any time but choose to limit my edits of my own posts to the same time as other users have just out of fairness.