The goal of a spec is to create a vivid moviegoing experience in the reader. Citing a specific song that’s vivid for the writer almost never does that (suppose the reader doesn’t know or like that song, e.g.). Using a generic description (“Mary’s boombox plays an upbeat reggae tune”) forces readers to call up a song or style from their own experience and thus involves them more directly.

Amateur writers often create specific soundtracks (ditto for verbose, expository action paragraphs) because they’re trying to put the movie in their head onto the page. The resulting script almost never creates a moviegoing simulation for readers, at least in my experience.