You're still utilizing the trailer to determine if the movie is worth your time. The main difference is that you're "forced" to experience them at a theater (unless you get there late). I (and many of my friends & colleagues) look up trailers for movies all the time, and we also download game demos from Xbox Live / PSN / Steam. The point of a game demo is (or was, or should be) to build the same type of hype for a game. It gives you a small taste, and hopefully entices you to pay for the entire thing. Some demos really do help a game and get the name out there. Other demos do more harm than good because the game itself isn't that great. This can happen with movies as well.

I believe the big difference between game demos and movies is that games are both longer than movies and much more expensive for the consumer. If you lost $8 on a movie on a Friday night, it might not be as big of a deal as losing $60+ on a game. Even if the movie sucked, you could still have a decent time overall (making the movie a small part of a larger evening). It's a relatively quick experience. Many people buy games hoping they'll provide much more than 2 hours worth of entertainment. If the game is terrible, paying that larger price seems like even more of a loss.

Sometimes I'll see a movie if the trailer is bad. I'll almost never buy a game if I didn't enjoy the demo. The more expensive the entertainment, the more critical people are about it and the more stingy they are with their money, at least in my experience.

-Chris