If there are buttons, at least you can memorize button presses, and often exit out enough times where you can get back to a known location if you mess up. With a touch screen, it is very hard to eeven know what places to touch, unless as you said we designed some kind of tactile overlay. Still, it is harder to accidently push a button than it is to accidentally push something on the screen without knowing it. At least if you push a button, you have the tactile sensation of the button being pressed. With a touch screen, I might accidently activate something and not even know it while feeling around for the place I meant to push.