All that said, I think you would be better served by VIEWS than by separate libraries.
...
There are some good reasons to use different libraries for different things. Just segregating different kinds of music is not normally a good reason. This is one of the things views were invented for!
Yes. A good rule of thumb is to ask yourself this question:
Is there ever
any circumstance that would make me want to (a) move information from one Library to the other, or (b) switch back and forth between these Libraries to accomplish any task?
If the answer is yes, or maybe, then you should NOT make a new Library, and make custom, filtered
Media Views instead.
If the answer is no, and you're sure it will never turn into a Yes, then go ahead and make a separate Library.
The best example of an appropriate use for separate Libraries I can think of, and what I use them for myself, is to have a Home Library (of all of your "entertainment" media) and a Work Library (which contains no entertainment media at all, but only stuff you've created or use at work, like stock footage, videos you've edited, sound clips if you're a musician, etc). These two "worlds" of using MC will never collide, and you won't need to replicate views, playlists, or move files in-between the two Libraries.
Using Libraries for things like separating different types of music, separating different Users, or separating different "categories" or "genres" of things is usually asking for trouble down the road. For some people, it might work out, but don't come crying 2 years from now when you want to merge them in the future (hint, you can't). As Brian indicated above, that's what
Media Views are for. Since Media Views
are searches, they can filter your media in basically any way you can imagine, so you only see what you want to see in each section of the
Tree (or Theater View or whatever).