I'm not sure pure CSS designs offer much in terms of usability and accessibility directly. It does help mobile devices, but I think that's still too small for many sites to matter. Plus with more devices that have full browsers, this will become less important.

But another side-effect of many CSS purists is also being an HTML purist. This leads to good semantic practices all-around.

Things like properly annotating title/alt properties. Or attaching checkbox fields to labels. Making sure the tabindex is in an order that makes sense.

As you said, with practice, everyone learns the benefits of each style and develops their own personal medium.