Once you start splitting the music sections, it could get messy. Anyway, what is "pop"? Is it just teen-friendly cheesy chart stuff, or is it anything that's not easily put into other pigeonholes, such as classical or jazz?
I agree that most of the sub-categories are little used, with most people sticking to the main headings. Best keep things as they are, I say. The whole point about a "music" category is that you can post questions about any sort of music in it. The majority of questions are about "pop" because that's the musical form accessible to most people ( lest we forget, pop is an abbreviation for popular after all). There is nothing to stop people posting about any other musical form though.