1. It's not used exclusively...sure. But, it's one of the knife's main tasks. Yes, I've seen a yanagi used for katsuramuki. I've also seen a deba used to slice sashimi...blasphemy I tell you.

That doesn't mean it's very good at it. The Japanese most definitely have knives made exclusively for one thing. A Unagi-Saki is made exclusively to prepare unagi (eel). There are actually several knife designs used solely in the preparation of unagi. A soba-kiri is made exclusively to cut soba noodles. You could certainly cut soba noodles with a usuba...but they make a knife just to cut soba noodles. A fugu-hiki is designed to cut fugu (blowfish).
The point is that you could use any of these knives for other things, but they are designed for a specific purpose.
1.5 Sure they do...see above.
2. If the back of a knife like a usuba is flat (I might have to go find some drawings for this....we'll see) it's impossible to sharpen as you have to grind the entire back every time. The back of the edge has to be perfectly in line with the back of the spine or the knife won't function properly. If the knife back was completely flat (like I see a lot of sort of non-traditional single bevel Japanese style knives made) most people sharpen a bevel on the back side of the knife. That pushes the center of the edge away from the back and more to the center of the mass of the knife....like a 50/50 ground knife if you will. This makes the knife cut poorly for it's intended purpose. The knife will not cut true and will actually steer.
Sure, you can make katsuramuki with a gyuto/chef knife....but making it with a usuba is much easier and part of that is due to the fact that the edge is where it is.
3. Stacy is a smart guy, but I don't agree with that statement. I've NEVER seen a flat back usuba. They may exist though.
4. That Takeda knife Gator is talking about is not single bevel. It's a double bevel knife.
Now, this is going to tax my brain cells a bit and I could be remembering wrong....but I believe a funayuki is traditionally single bevel and is quite thin....I believe it might have been the basis for the Suisin Momiji knife I mentioned. It would be the closest to a hollow back, single bevel santoku.
I didn't proof read this, sorry. And I need to get back to work....I'll add more later.
Edit: Picture:
http://www.knifeforums.com/uploads/1200090538-Knife_Diagram__traditional_Japanese_.jpg
I'll explain later.