The other reason not to have favs auto-upvote is that we already have order:fav for people who care about that.
(Incidentally, does order:fav still only count favorites by privileged and above users? I always thought that that was a bit ridiculous -- I can understand restricting score, but what's the point of having order:fav display more or less the same things as order:score? It can be useful to use order:fav see what the whole population here likes, even if it's just porn.)