BuddyPress.org

@modemlooper – roles would restrict you to just one level of categorisation

Category and tag taxonomies for members would allow you to implement much more complex groupings, e.g. hierarchical nested groups of members, allocation of members to multiple categories, etc.

[Edit: This argument applies to groups as well as members, and is something Burt Adsit tried to achieve in the early 1.0 BuddyPress days]