Because the adds themselves almost automatically suck. Also, I don't like level design being centered around displaying billboards rather than gameplay, which is a real risk, in for instance multiplayer shooters.
if it negates some of the costs involved in these big titles and keeps us from paying more and more for a game, then I'm all for it
I'm curious. Do you really think we don't already pay exactly the amount they can get away with charging without hurting the games general distribution and popularity. Adds might help finance a game in some small way, like it does a movie, but it's not like the retail price is going to change.