I don't get comments like this. It would take you 2 seconds to test and realize that you can touch every point on a touch screen laptop without raising your elbows of the desk. I can touch at least the bottom third without even moving my elbows from the position they naturally rest at while I'm typing. In other words, on a theoretical touch screen macBook, I'd be able to fully interact with at least the dock without making any changes in my hand/elbow position.
Sure, you can slide your arms / hands / fingers forward toward the screen, with elbows on the desk, but it is a very inefficient movement and if your entire work day revolved around doing that, you'd likely develop tennis elbow very quickly.
I'm sitting at my desk now, in fact, and my normal hand/finger position on the keyboard does not allow me to even touch the screen (not even at the bottom where the dock is), without having to slide my arm forward. The only movements my elbows / arms make are very small adjustments if using the trackpad, otherwise my wrists are resting on the wrist rest of my 15" rMBP and I can work uninterrupted for hours in this position. The same is true when I'm standing at my desk (a sit-stand unit).
I've mentioned this before, that I tried to make an iPad and BT keyboard work similarly, for travel instead of bringing my rMBP with me, but it is just so inefficient, having to move between keyboard and screen, I stopped trying and went back to either bringing the laptop or just the iPad (using on-screen keyboard).