1) Music is superlative and creates all kinds of expectations regarding the movie. Movie does not live up to the expectations and both movie and music album perish under the weight of the unsuccessful movie.
2) Music is superior, movie not so much. But that does not put a dent in the success of the music album.
3) Music is sub-optimal but the movie is worth watching. You pretty much sit through the songs to get through the movie experience.
4) Both movie and the music album suck equally. Not much to say here.
Good news is that there are more movies in the first and second categories and fewer in the third category. At least the music directors are putting some effort. My problem is with the first situation. That is plain unfair to the music album. Fitoor and Black Velvet come to mind. I can live with the second scenario as my interest is primarily in the music aspects.