I imagine most people won't bother giving reasons, but at least they'll know the post has been edited. If you read the last post in a thread then go back when someone replies and notice that the 2nd to last post has an edit note on it, you can re-read it to see what's changed as it might have changed the meaning of the post.

For instance if I was praising Pascal,
and follow on posts disagreed,
so I decided to edit out my praise, giving Reason: Bad grammar.
At the moment you can do this for up to 24 hours and there won't even be a note that you edited the post. Before, you could do this anytime you wanted, so I think we've made an improvement. I think we can have it so moderators/admins can see past versions of posts before the edit, so if someone did call you out on changing your post we could verify it.

I don't think a moderator for a single thread is really going to be necessary in any case. If you want to edit the initial post, just PM me the edited post and I'll replace the original for you. Giving moderator powers to people who don't know mod procedures etc opens up problems which MMM quite rightly pointed out.

Also, I said from 1 minute after because I make plenty of tiny grammar mistakes that I only notice when I read it after quick replying and then quickly edit. If you change the post within 1 minute it's more than likely no one has seen it anyway, and it's hard to change anything substantial in that time frame.