one would need to spend a lot more time for tagging and subsequent search string composition for each library query.
you could easily make autoplaylists based on File Path to filter your library (no tagging involved).
Not sure what you mean by search string composition but, if you keep your stuff in different folders, I think you could easily make autoplaylists based on File Path to filter your library (no tagging involved). Or am I missing something?
at every place <snip> you will have to consider the mixture of your library.
I still fail to see how having multiple libraries (however it is implemented) can fix that: you will have to specify in every place which library to use. Unless you are suggesting that one has to choose a specific library at startup.
Just how it could look like: some master filters are defined centrally in library preferences and from a library viewer inside one tab i assign one of that master filter to that ui-element: so we could have one tab for audiobooks, one tab for music, one tab for podcasts, one tab for download folder, one tab for speeches and so on.