Some recent examples include "Avengers Assemble" where only the Soundgarden song was in the movie, and that was the ending credits song, "Songs From District 12 and Beyond" from the Hunger Games, and "Almost Alice" from Tim Burton's Alice In Wonderland. I don't know about you, but I don't usually buy these compilations.
I notice a great score, and also a great soundtrack as much as the next person. Soundtracks have completely shaped the way I think of certain movies. What would Blade Runner be without that Vangelis score? Star Wars without that John Williams score?
Trainspotting, Pulp Fiction, O Brother Where Art Thou, Party Monster, Basquiat, Natural Born Killers, any Wes Anderson Movie or any John Hughes movie- these are some of my favorite "compilation" soundtracks. The songs that appeared in those movies completed the scenes they were used in, and I will forever link those songs with those movies.
It just feels somehow a little cheap to me to see songs "Inspired by" a movie that aren't part OF the movie. It just comes across to me as another movie tie-in product to have on the shelves. I get the same "eye-roll" reaction when I see a book that's been turned into a movie and suddenly the (possibly great) book jacket has been replaced with a picture of Brad Pitt or whoever. Is that necessary?
Iconic soundtracks are iconic for a reason. I don't know how many people would care about songs "inspired by" Saturday Night Fever like they did that incredible soundtrack.
That's my 2 cents as both a music and movie lover that appreciates the magic of the stars aligning to make a great movie have a great soundtrack.