Disclaimer: I've been a musician most of my life and I probably study music a bit more in movies than most people. That being said I think great movies and great composers don't give anyone a choice about listening to the score or soundtrack. They make them feel it.
Great scores equals great movies! Or at least the movies MUST have great scores to be great. Fair? I think so. I've been hooked on the movie Inception lately I think it's great. I keep replaying the music in my head, and along with the music come the visuals reaplying through my head. Hans Zimmerman did and incredible job as he did with Batman Begins, Gladiator and the great soundtrack for the DaVinci Code. John Williams, Hans Zimmerman and Danny Elfman are three of the best composers of modern movies. If you doubt it look them up!
I really do believe that the musical score as at least 50% of any multimedia visual storytelling. Remember when you put something together that involves a musical element it most flow, compliment and be part of the story. Not just an afterthought!