I'm not joking, I think the distinction offered by the Academy between score and soundtrack is absurd. The use of songs, original or not, should be considered soundtrack and orchestration, original or not, should be considered score. Like FLY says, these two things serve very different function within a film and the distinction of originality has nothing to do with that function.
Well, the Academy probably makes that distinction to promote the use of original scores and not have long dead composers nominated for score. I don't think they're making an artistic distinction and it's not like they have an award for best soundtrack that accepts non-original compositions.
I do agree that from the perspective of watching a film the difference between original or not is not particularly relevant and should stand on the artistic merits of how it fits into a film.
That said the only soundtrack or score that I remember and have positive associations with are Badlands and Drive. If I notice a score or soundtrack then it's no longer complementing the film but overtaking it and, except for rare instances, that's not a good thing. Also in the case of Drive, even though I just watched it a couple of months ago I could not tell you if there was an actual score beyond the songs.