The only thing I use a second library for is my holiday music. The reason is that if I put it in my regular library, then I have to go to a lot of trouble to filter it out of all my playlists, filters, searches, etc and it's not worth the effort for music I use about 1 month a year.
Aside from that, new things go to the Inbox, and non-music goes in Audiobooks. Everything else I use filters and playlists for. Classical vs pop, various genres, what to sync (this I also make use of the tick boxes for), things that need more processing (combined with virtual tags that identify missing info), etc
In general, working with multiple libraries causes more problems than it solves. With filters or playlists you can change what section a file is in by changing a tag that it's based on. There's no good way to pass files from one library to another, and they share all the same settings except the stuff in Library Preferences, anyway. (If you need different auto-organize settings for different things, you can use exceptions to create multiple templates.)