I'm often at the receiving end mixing the music with the rest. I much prefer stuff that isn't too slammed and it usually sounds better on air. Sometimes there are surreal situations where a big budget slammed hit is followed by some unknown library track and the library track ends up sounding cleaner on air because it gets through the processing much better.
I doubt level plays too much of a consideration when the editor/directors chose the music. They are looking for vibe and atmosphere to support the on screen action. Levels are rather irrelevant. In the unusual cases when music is added in audio post (rather than video post), none of the libraries I hear (whether online or on CD) sound slammed in any way.
On the other hand, I have sometimes requested that a commercial track be replaced because it just sounds so bad because of the slamming... You don't want that happening with your library music now do you?
Alistair