The bigger your game is, the less you need a demo to sell it. Demos make great sense for small niche games, unknown firsts and games that just doesn't sell very well.
But what purpose would a Gear of War demo really serve? Everybody knows about the game, they know how it looks, what it's about. The risk with releasing a demo for games as heavily frontloaded as Gears of War or Halo 3 is that the people would have bought the game day 1 just because of the hype will try the game and not like it.
You could look at it like this, 10% of those who try a demo (I made this figure up) is going to buy the game. The demo is good if less than 10% of those who tried it would have bought the game anyway. If you have a big game, the amount ready to buy the game before trying the demo would be much higher, and it's much more difficult to make it a good business move.
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