Sure there are a lot of garbage ef-s lenses that won't work on a ef mount but why would you want to use an inferior lens on a good body? The L lens line covers the focal length and is far better than the ef -s line. (Excluding the 17-55) that's a great lens. Sure you don't get the ridicules focal lengths like 18-200 but who wants that on a full frame anyway . I'm surprised that you even bring this matter up. At the end of the day you get what you pay for and that's why the ef -s line is do much cheaper .
I had a Sigma 17-70mm which, if it worked on my 5D, I would still have. It's an EF-S mount and, in my opinion, it outperforms just about anything that Canon produces is the same price range.
You ask why someone would want to use an "inferior" lens. One reason, and it's the first reason anyone considers, is cost. "L" Series lenses are not inexpensive. Yes, they produce wonderful image quality, but that image quality comes at a very deep price. Also, an "EF" designation doesn't necessarily mean that a lens is of super quality. Take a look at the Canon 50mm f/1.8. It's not an "L" Series lens, is under a hundred bucks, and produces perfectly acceptable results. They're not going to blow anyone's skirt up, but they're fine results nonetheless. On the other hand, the 85mm f/1.8 also isn't an "L" Series lens, is also an EF mount, and produces ridiculously fine image quality. It
should have a red ring on it.
Your post also suggests that, since you believe a full-frame body is a "good" body, that you believe a crop body is, somehow, inferior. I own both. Both perform equally well in their respective tasks...