Can anyone imagine what a touchscreen would look like after an hour or so of typing on it? It would be so dirty!
I can, I have one.
It doesn't.
When you seen an Iphone, is your first reaction - "Ick, the screen is dirty"?
One big reason is that the system has a keyboard - and if you're going to be typing for an hour you'll be pressing keys.
The other is, that like the Iphone, the screen coatings and brightness minimize any perception of dirty screen.
I wouldn't eat fried chicken with my fingers while typing though, that's too much.

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The touchscreen on a desktop would be more useful for the more passive activities like reading and surfing.
A touchscreen laptop is more natural - your hands are already close to the screen.
I've noticed, after many months of Windows 7 multi-touch on my laptop, that I have more fingerprints on my LCD monitors than I ever had in the past.
I'm engaged in what I'm reading, not resting the hands on home-row and the mouse. I get to the bottom of the screen, and the default motor response is to poke the "next page" button on the screen. It does exactly that on the multi-touch laptop, but on the 8-core it just leaves a smudge on the big LCD

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When Apple starts to sell multi-touch laptops and Imacs you'll understand too. It's a big step in UI.