I remember the broom exercise at school. Funny, isn't it? Big strong lads can't do it for more than 45 seconds. I thought I'd try it myself given that I'm not 12 any more. I got bored after 5 minutes. Sure, my shoulder aches a little bit, but I reckon I had another 10 minutes of holding my arm out, stock still. (I think it's stretching the facts somewhat to suggest that limpwristed athletes represent the epitome of health.) But touchscreens don't demand that, do they? The "stock still" bit, at least. When I was draughting (that's right, just before CAD took off), my arms would get tired. My back got tired. But I could do my job for a full working day, because of the motion.
At a Desktop computer, you're not constantly reaching for the mouse. And then consider that Windows 8 simply isn't designed, nor meant to be used exclusively with touch at the Desktop. After all, it runs nicely on machines without touchscreens. Why is mouselessness seemingly so important to people who are sworn off Windows anyway? (Not that I'd have the bloody thing in my house.)