I'm not sure if that's only true for sci-fi movies, but it's clearly true for several of my all-time favorites: Bladerunner, Alien, 2001. I actually watched several movies and tv shows just because I was interested in the soundtrack or musical score, e.g. There Will Be Blood (Radiohead's Jonny Greenwood), Les revenants (Mogwai).
It's also the best part I remember about the first 'The Crow' movie - I don't know how often I've listened to that soundtrack. It even led me to discover several bands that are now favorites of mine.
Unfortunately, too many movies and tv shows overdo it in the musical department. I recently watched a movie which had background music or songs in every single second. This was incredibly annyoing after a while and actually distracted me from the movie. Another bad example, at least for me, are the Lord of the Rings movies. I. just. cannot. watch. them. anymore. And just because of the musical score: if I never hear 'Frodo's' Theme' again it will be too early!
"Sound" not "score". The score is half of it, but I was also talking about the wail of a TIE fighter, the sound of a proton pack powering up, the TARDIS materialising - sounds which become iconic on their own.