I think that facets, tags and category trees are just different expressions of basically the same paradigm. Facets are tags organized in groups with some enforcement related to specific information types (one-of, many-of, required facet). Strict category trees are tags hierarchies which only allow one tag per object.
The distinction between the three is made with consideration to UI, performance and data structure in mind, but I don’t see a reason to treat them as completely separate entities.