I agree that trailers these days often ruin plot points or particularly effective moments that would be better seen in the actual film, but I still enjoy trailers.
For one thing, I love thinking about the quasi-artistic process that must go behind making them. Like, how do you convey the basic premise of a film and make people want to see it in the span of two minutes? Something like the Cloverfield trailer is an example of when the trailer reaches a level of being entertaining and exciting in and of itself. This didn't detract from the actual film more than just working too well and generating more buzz than the film could live up to.
The excitement is a big part of it too, at least for us nerds. I for one will wholeheartedly admit that I love the endless years of build-up before an event film, especially when the film pays off. Seeing The Dark Knight after all that anticipation and watching every bit of information released about the movie was like a ridiculous, long-delayed, nerd orgasm. And I can still remember spending hours trying to watch the Batman Begins trailer, my first glimpse of the movie, at frame-by-frame pace as my dial-up connection struggled to play it.
Most importantly and broadly, I just don't think you're looking at this logically, Planet-Man. The reason why people like Roger Ebert can hate trailers is because they're critics and they at least have to try to be objective in how they watch films. They can't buy into pre-film hype, or be influenced by it. Not to mention the fact that they already see most of what comes out, because that's their job.
Regular people have to actually choose what they want to see this weekend, and as romantic of a notion as it is, most people don't want to just walk into a theater and see a movie that they know nothing about in advance. People would base their film-watching decisions off of the people involved or the genre or whatever, and would actually become more limited in their taste. No one would have any reason to try out a movie that doesn't already fit their pre-designated ideas about what appeals to them. When I was 12 I saw the trailer for 28 Days Later in the theater and nearly had to leave. I didn't know who Danny Boyle was or what "Trainspotting" was or who any of the actors were and I hadn't even really begun to like zombie movies yet. I just knew that the movie looked like the scariest ****ing thing ever. Six years later and it's one of my favorite movies, the best horror film of the decade in my opinion.
The point is, trailers are advertisements for products, designed to appeal to audiences and encourage them to purchase the product being advertised. If you want to swear off trailers, that's cool. But you can't expect normal people to do the same when they don't have the time or money to spend on something they know nothing about.
Also: if you're looking for great trailers that don't spoil anything about the movie while generating sufficient interest, then this is perhaps the best -
[youtube]I6qDqdYY6-Y[/youtube]
Imagine what it must've been like to see that, how terrifying and mysterious the movie must've looked.