You are experiencing a 'compilation score' (see but distinguish from 'source music') in a motion picture where the 'background music' is, for the most part, cobbled together out of pre-existing music material. Now-a-days, this pre-existing material is usually 'popular songs' (note Natural Born Killers); however, strictly speaking, any film score which is made up, in whole or in large part, from pre-existing music of any kind qualifies as a 'compilation score' - note 2001: A Space Odyssey  where the background music was a 'sampling' of various materials from our long established 'classical' reliquary . But such true 'compilation scores' are actually rather rare. The usual case is where the background music in a motion picture is in fact mostly original underscore, in the traditional sense, but with one or a few popular "songs" mixed in. Whether a compilation score is or is not 'effective' really depends upon which camp you belong to or sympathize with among the many that make up the movie making enterprise.

    Producers and studio marketing executives often push for hit songs to be part of a film's score to achieve a 'cross-pollination effect' - i.e., a popular or hit song in a film can tends to sell more movie tickets and a film with 'pop songs' can help sell more records... oops, CD's. The Bodyquard and Waiting To Exhale are sufficient evidence for this proposition. But generally, film composers, and often directors as well, seem aligned against the use of songs to displace traditional film score on artistic grounds. Even Danny Elfman (Batman, To Die For, Men In Black, etc.), who entered the film side of the music business as a popular recording artist (Oingo Boingo), has his qualms:

    Wherever you stand, one can't help but appreciate that it is this kind of tension between the "suits" (marketing types and studio execs) and the "creative 'types'" that makes the movie business the interesting and fertile area of endeavor that it is.