(By the way, I'm afraid that there is less movement (and less eyes) in this forum than in tikiwiki-devel list)
For what I understood, category permissions are just to make easy restricting access to many objects in one category node (or branch) on view objects and edit objects (like, to make kind-of-intranets easily on specific content, assigned to a category/categ. tree), but they were not intended to substitute the more detailed and fine grained object permissions (per type of feature).
Anyway, I've heard some people complaining about the categ. permissions, and even read some people suggesting to remove then in Tiki 3 (!), but as far as I know, this has not been publicly discussed nor agreed.
So, you might search the tikiwiki-devel list for "category permissions" or so.
And as far as I remember, Louis Philippe (lph) had something in mind to a future rewrite of the permission system... so why not starting the thread at tikiwiki list, if you have some ideas of how to improve/rewrite that system?
P.S. Still wondering how good your home-made italian wine must be...