I did once have segregated Calibre libraries. One for cookbooks, one for technical books in PDF and one for 'regular' books like novels and nonfiction in EPUB.

The only reason I had it like that was so I could sync just the cookbook files to another device via Syncthing, or only the technical books. It didn't last, it was a pita to manager (three different auto-add folders) and I reunified them after 9 months or so.

Being able to switch between multiple libraries under one application, and being able to assign those libraries based on a logged on user is sort of what I'm talking about. My partner and I have two very different tastes in books, and managing them under one library is a pain.