He clearly makes a point of supporting user CSS, but also a point of being usable by default, so it clearly misses that...

Also, I doubt most regular users could be bothered to actually write custom CSS for the entire web. Most people have zero knowledge of how to do it, not to mention how to actually improve what they want to improve.

The only reasonable real world case I can see is accessibility, which is certainly a valid case, and should be considered by all web designers/developers.