[marr]

One problem would be that at best you still have a finger blocking some screen content, and at worst your whole hand is blocking. Is something in the upper left corner an "on-finger-over" that brings up a chunk of text you need to read? Uh oh.

Another would be constantly having to make the effort to not lift your finger(s). Need to reach for a pen so you can write down what you are seeing on the screen? Is that a "let-go" or a "hang-on-a-second-pretend-that-didnt-happen" gesture?

These are limitations of touch interfaces. Touch is not panacea; it's the finger painting of the computing world. That's why it appeals to those who need simple.