Visibility. Touch screens are annoying because your hand blocks your view.
http://www.newscientist.com/article/dn12773
They put "dots" at the touch point so you can see exactly where you're touching, unlike an iPhone.
Neat concept but you need some sort of camera or high resolution sensor for tracking finger movements when they're not in contact with the backtrack....otherwise you will need a "cursor" displayed on the screen which is also visually distracting.