Categories? We don' need no stinkin' categories!
For me, a problem arose when too many categories were identified. Many of those were actually sub-sets, not stand alone for example. Some viewed blues and rhythm and blues as individual identifiers, some put them together. Others considered blues as a sub-set of jazz while calling R&B a sub-set of R&R. Then within blues you can find Delta, Chicago style, Detroit style, urban, country, East Coast, where does it end? Further, Chicago may have Southside and who knows what others?
For me the only time defining categories is meaningful is when arranging my music collection, and that is to simplify by major musical type and then sort alphabetically to facilitate finding what I'm looking for. So I defined eight categories for that. With a smaller music library I could live with fewer categories.
Musically I agree with Duke as I quoted recently, but that is not helpful when searching through hundreds of choices.