People might try to do that just to get their own videos promoted without using up a point (or if they aren't ranked high enough to promote). For example, submit a vid, then submit another copy of the same vid, then dupeof the second one in order to get the first promoted.

Well, you'd have to be pretty sleazy to pull a stunt like that, plus that type of abuse is very noticeable and can be dealt with.

That said, I think this is a pretty good idea, though there's already the problem of there sometimes being way too many promotes on the front page, and this'll just make that worse.