In the 360's case, they typically do not release demos for their bigger games. Correct me if I am wrong, but Halo 3, COD4, Gears of War, Oblivion, Mass Effect, and Assassins Creed did not have demos. When a game has a massive marketing budget, I guess they really do not see the need when all it can do is hurt them. For games like Bioshock, Crackdown, and Viva Pinata though, these are games that had little mainstream recognition so good demos probably helped them out. I think it can only hurt a game when it is highly anticipated and the demo is shit. I think these numbers are going to be a bit skewed just because the bigger releases that the companies know are going to sell tend not to have a demo.