What is the advantage of EF-S glass?
I have heard that crop sensors have better focus in the middle of the lens on an EF mount. Does this mean it can take full advantage of the entire lens in an EF-S type mount?
The original rationale was lighter and less expensive. When I first picked up the 10-22mm EF-S, I was surprised how light it was. I like that. Personally, I'm not willing to invest in full-frame bodies due to price, and I find that several of the EF-S offerings (such as the 10-22mm and 17-55mm) are great lenses.
As far as the second part of your question, I've never heard the term "better focus" used, but it is true you are using the center cut of the lens, meaning if the lens has soft corners or vignetting, these attributes will show up less on a crop sensor compared to a full-frame sensor.
As far as an update, I don't feel like my 10-22mm needs any better IQ. I find it to be better than my 24-105mm and in the realm of my 70-200 f/2.8L IS II. The 17-55 would benefit from better construction, however, but I, too, am worried about the price inching up. Conversely, a II version could mean cheaper I versions, so ...
Like Thom Hogan on Nikon, I'm still wishing for a fast 28mm or 24mm equivalent prime EF-S. This is my favorite focal length, and f/2 or faster would open up some opportunities for me.