Some people enjoy going into a movie cold, whereas others (like me) enjoy knowing what the film is going to be like and reading in depth reviews before going in. There is nothing worse about the movie watching experience to me than going into a movie cold and discovering that it's something that just doesn't appeal to me in the least or is an irredeemable piece of dreck. Knowing things about it in advance allow me to minimize the chances of its not appealing to me or being dreck.
Trailers can be good or bad, revealing or not, and sometimes quite misleading. I don't seek to avoid them altogether, and sometimes a trailer is the only way I hear about a film I'm interested in seeing. I agree that some give away too much, but frankly I would rather err on that side than to be mislead about what a film is going to be like on screen from a trailer that makes it look good when it isn't.