I know books can belong in multiple collections -- most of my Kindle books are.
I don't see why that obviates the use for or need of a not-in-any-collection view. As I already said, I consider it a fairly intuitive need.
There are plenty of books that do
not belong in any collection at all. I find it extraordinarily helpful to be able to see at a glance which those are. My Kindle can do it.