I don't have time to make changes like that "for the sake of it"; the goal is always to make the site better and increase participation.
In general, with any forum, you want to have as few sub-forums as possible while maintaining organization and usability. With fewer sub-forums, more people are exposed to more topics and more users, which in turn fosters more discussion and a greater sense of community. Conversely, if there's not enough sub-forums, users can be discouraged from discussing particular things or going in-depth on a specific topic.
While an experienced, long-term user like yourself might prefer a more organized structure and claim that it was "perfectly fine the way it was", you don't really know that. Music, movies, and TV are very closely related (if you're interested in talking/reading about one of those subjects, you're probably going to be interested in talking about one of the other ones, too), and more users may be more likely to find some interesting discussions here they otherwise would not have seen. I'm happy to trade organization for activity, and the only way to find that out is to try it out.
It's a delicate balancing act, of course. There is no real right answer, but the hard data can help you get close. So far, traffic in the newly combined forum is up over 33% from the traffic totals of the two separate forums.