However you swing it (he he), you've still only got one position where your wrists and fingers can be both productive and comfortable: resting almost horizontally with palms down on your work surface. Mouse, keyboard, tablet, or pad of paper, it's all the same. The tablet has to replace that spot where the mouse and keyboard would be—I don't see how they can coexist efficiently.
And you'd have to be looking down all the time. Multi-touch computers are an ergonomic nightmare, because your hands and vision have to be directed at the same point. Either your hands have to be raised up, which is tiring, or you have to be looking down at your hands which is problematic for your neck.
Many of the things we do on a daily basis don't require us to look at our hands, eg: driving, brushing teeth. Some do, eg: cutting food, but we don't sit in one spot, the one position, doing them for hours.
I'm not saying multi-touch isn't cool and useful, but it's only suitable for certain applications, like iPhone; it's certainly not suitable for general computing use or long-term device operation.
How many people really type? I think us touch typists are in quite a minority.
Based on....? They still teach typing. And even if you don't touch-type, that doesn't make things any better, because you'll be staring at the keyboard all day, since that's also where the screen is (ie: you won't get that look up, look down, look up, look down variance). While typing in general is not great for you, touch-typing is ergonomically better, because it doesn't involve unnatural body positions.