I find that using separate libraries is most useful/successful if you use them for totally separate types of media that will never need to be used simultaneously. If you arrange it that way, the time needed to switch back and forth isn't a serious issue because the need is rare and it matches with your workflow (I need to start a new and completely different kind of task, so I need to switch Libraries).
For example, I'm a video editor at work. I have a large collection of general "stock" material and project-specific media (audio, images, and video) that I use for these projects. I have a MC Library that I use to manage all of this stuff. I also have a MC Library that manages the server locations of all of my "completed" video work that we have published on our webservers at the office (both internally and externally). I use those MC Libraries to organize the files, tag them, and allow me to search through either the "published video" we have here at the office, or my "source material" when I'm building something new.
But both of those uses are completely separate from my "personal media" library (music, tv shows, etc), and I really don't ever have the need to use these two sets simultaneously. (Well, rarely... Occasionally I want to listen to some music in the background while I work on a project, but I have multiple machines everywhere, so this isn't a big deal, I just open MC on a different machine and connect to my personal library there. Generally, I can't listen to music while I'm cutting though, so even this limitation is very infrequent.)
However, these are, as you can see, completely different "types" of Libraries. One is my personal media collection. One is a tool I use to build media projects. And one is used to manage completed, unchanging, published video files (which are in a totally different format than what I'd use as sources). When you have this type of division, that's where I find it is often most helpful to have separate libraries. I could, of course, combine all of these into one Library, and just use categories to segregate everything out. But, from a use-perspective, it is nice that my "Editing" Library has a small set of views that are all specifically tailored to the job at hand, without a bunch of "noise" from my Personal media library or the published video. Plus, with many of my video projects, it is very important not to move assets after they've been used in a project (because that breaks the links to these files in all the projects where they were used). Having a separate library for this stuff makes it easy to manage when files can be moved/reorganized, and when they cannot. Likewise, my Published Video library at the office, I can be sure that any file imported in there is a live, active video that exists somewhere on our HTTP or Streaming servers. If it is in there, it is published, and if something gets in there that isn't from the right place (something gets accidentally imported from the C drive or something) then it can be safely moved or deleted without impacting my published video.
If, instead, your goal is to sub-categorize different "styles" of the same top-level "type" of media (all personal music) then it will likely be much more trouble than it is worth to separate it out into different libraries, and using the existing fields, or a custom field if the existing ones don't work for your needs, would probably be a better solution.