Not to be curt, but it clearly isn't obviously useful—otherwise the project wouldn't be languishing such as it is. The notion of creating a single overarching conceptual map to regulate the representation of the varied manifold of human experience on the web is almost certainly a deeply misguided idea, and even if it's philosophically sound (a big if), it's not clear that schema is anything like the correct approach. I'm open to be convinced of it's value if you'd like to elaborate, but I'll just say its far from obvious.