Sorry to interject, but I run into the questions you posed. I listen to a lot of soundtracks, and people ask me how I can listen to them as music.
I believe you are correct that some composers create background noise and some put enough effort into the music in order to allow it to stand alone with it's own character. I prefer to listen to the latter as a means to spur creativity with how the music takes me out of my normal comfort zone. I like to write fiction, and listening to a soundtrack without having knowledge of how the music was used to create my own stories.
Unfortunately, I do think IS can stand on its own when complete, but sampling, as it was used in the movie, turned it into background noise. In all honesty, I prefer Unsustainable over IS.