Try using a touchscreen laptop for a while and tell us how much it sucks pretty quick. Jobs even talked about the fatigue in one of his keynotes. Holding your arm up like that for extended periods is no fun which is why we haven't seen it in places like the iMac even though outside manufacturers have offered touchscreens for the iMac for years. http://www.trolltouch.com
Plus the bigger the screen the farther away it sits... Your arm would get very tired on a iMac. Screen being farther away so as not to burn your retinas...lol. Touchscreen on a laptop or desktop is overrated. It was just a logical step till everyone went ape **** over tablets. touchscreen laptops are like 3d TV... (yawn)