I love film trailers. I get so excited when the first trailer for a film I'm looking forward to comes out. I think the Beauty and the Beast teaser is brilliant. As was the first and second Star Wars: The Force Awakens trailers, and even though I've normally already seen all the newest trailers, I hate being late to the cinema and missing the ones before a film.
They have certainly become an event of their own.
A good trailer shouldn’t give away the entire plot. It’s an amuse-bouche. It should entice and excite us, not make us feel like easily exploitable consumers. The great B-movie producer Roger Corman used to let his directors make whatever the hell they wanted, so long as they gave him enough bare flesh and explosions to fill a 90-second teaser. He knew that cheap titillation was a sure way to sell a film, but he also cared about how each of his films was sold. Now, with trailers growing ever bigger, brasher and more hyperbolic, it seems the movies themselves have somehow gotten smaller.
http://lwlies.com/articles/how-trailers-changed-the-way-we-watch-movies/