The problem
with music is that the writer doesn't own the music or the words.
Human nature is to assume that what ever is in a film is part of the film, that everyone agrees with the ideas of the film.
Singers and songwriters OWN those works and MAY not want to be involved with YOUR story. Regardless of the reason.
SO, you write in a song, the owner wants to get PAID for it.
The rights aren't likely to be available, or may be to pricey.
Hence, it is a "rule" that you don't have a song be vital to your story.
If you can't get the rights then your story is screwed.
If the song ISN'T vital to the story, then why bother?
It's a non issue. A generic ref will work.
Where did we get the idea?
From studio heads, readers, and pro writers who are in the know and have answered this question a zillion times on message boards.