You know, I don't get why this was never done. Back when categories were first introduced, I expected this to be an obvious thing. I expected there to be at least eight new namespaces: 'Category', 'Image category', 'Wikipedia category' and 'User category', plus all their corresponding Talk namespaces.
I think this would help a lot. We've already seen a lot of Categories and Templates that are used primarily for organizing users and user pages, so that we can find Wikipedians by language or interest. The CSD debates that I've been in seemed to show that other users were against users having categories for their user subpages (Category:User:Creidieki/Sandbox4 and related debates).
It would be nice to have a namespace distinction between the categories "Articles about Japanese people" and "Wikipedia users who speak Japanese".
-- Creidieki