This should allow WM to introduced a proper combinatorial search engine based on tags for >Commons; and many of the most egregious Commons intersection categories will wither on >the vine. (There is debate as to whether Commons will end up needing *any* category pages, >but I suspect it will, because they are just so convenient to use as places for jotting down >facts -- on the other hand, it is possible one might be forced to create an associated >Commons article/gallery for that).
Just for reference, the existing search engine can already do category intersections (via incategory:foo queries), and there are even tools to do category intersection when treating the categories as a transitive relation. Yet we still see "intersected" categories. I'm unsure if the structured data initiative will really kill them.
--bawolff