Another problem with game demos is that they can make the final game too easy. I’ve discussed before how games each have their own skill that’s obtained via practice, and can usually be applied to any level in the game. Since demos are often composed of a sampling of levels, of varying difficulty, if you can beat the entire demo, you’ve acquired enough skill at the game to beat the hardest level in the demo—without having had to beat all the intermediate levels. This causes two problems: Not only are the demo’s harder levels incredibly difficult for the demo-player (since you don’t have the benefit of approaching them gradually), but once you have gotten them down, when you go back and play the real game, it’s too easy.
Case in point: I just purchased Marble Blast Gold after having beaten the demo levels. I managed to make it through all twenty-four of the game’s “beginner” levels in under an hour, beating the “gold time” on many of them.