If multi touch on the screen of a desktop size machine was going to take off, it already would have, as there are several machines with this technology already available.
The problem is that it has not taken off, and probably never will. Sitting inches from the screen, moving your hands over the screen for minutes/hours at a time, and having a smudged up screen all equate to not such a great computing experience as you apparently think.
I'd love to see something like this though: http://www.techcrunch.com/2009/10/13/10gui-one-very-slick-desktop-multi-touch-concept-video/