I never understood this. Even if I was really looking forward to a movie and someone gave away the ending, I would still enjoy the movie because I watch them for the experience and the story telling. If spoiling a movie mattered, then people wouldn't own movies they've seen before.
But people like to be surprised when they see a movie, especially is something really awesome happened during that movie. And people rent/buy DVDs because those are they're favorites and they actually enjoyed the experience of watching that movie.