I am a member of Uni Legal Services and I would like to tag something as an "Opinion". However, I don't want other people outside of the Legal group to be able to tag their things as "Opinion", otherwise if I do a search for category=opinion, I will pull things that are not supposed to be in my "Opinion" set. A similar requirement is needed for category and aspects (i.e. control who can associate a particular category or aspect) with their things.
In my past work with repositories, we've come across metadata namespace concept that allowed ACL to be defined on these namespaces. That way you can have a category namespace say Legal:Opinion and only the Legal people are allowed to use the Legal namespace of category. You can set ACL so other people can be consumer of the Legal namespace to allow them to view that metadata.