Why so much info in the movie trailers?

I haven't seen the trailer for Avatar but if you look at classic movie trailers they give pretty much everything away. The trailer for The Apartment (1960, Billy Wilder) actually has the ending scene of the movie in it.

I noticed that the trailer for "Brothers", in an attempt to sell us on the story, explains the entire plot up to what looks like about ten seconds before the climax. I'm kinda confused about why anyone would pay $10 to see those last ten seconds to find out what happens to the three main characters.

But PS No one's going to see Avatar for the good plot! They didn't drop half a billion dollars creating a really bitchin plot. It's equal parts badass special effects and a massive raging ad campaign.

Movie trailers are a major pet peeve of mine. I hate having a movie spoiled and they have become progressively worse about this. With comedies, the trailers give away every single punchline. When it comes to movies I intend to see, I refuse to watch the trailers.

From a marketing standpoint, I guess detailed trailers must work because the studios keep using them.

Kathleen42 said:

I haven't seen the trailer for Avatar but if you look at classic movie trailers they give pretty much everything away. The trailer for The Apartment (1960, Billy Wilder) actually has the ending scene of the movie in it.

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I disagree. I watch a lot of Turner Classic Movies and many of the classic trailers they show there give barebones detail about the plot, relying mostly on the titles splashing up on the screen (ACTION! ROMANCE! ALL SINGING! ALL DANCING!) or hyperactive narrators to let the audience know what to expect. All About Eve has staged "interviews" with Bette Davis and Anne Baxter, talking about the titular Eve and her ambitions in the story, for example.

For the classic and older trailers, though, before the advent of TV advertising, the only place people saw these was in a theatre. They weren't oversaturated to the point of nausea; I'd pretty much memorized the trailers for Home Alone and when I finally saw it, I was sorely disappointed because they pretty much showed you the highlights in the two or three trailers they ran on TV, and I could've saved the $7.00 (at the time). If somebody had seen the trailer for The Apartment once about three weeks before they saw it in the theatre, or if they were only relying on newspaper and magazine ads, they probably wouldn't have realized they'd seen the ending.