You missed my sarcasm. I was replying to the poster's question of why you'd want to stop using a trackpad in favour of a touch iMac.
But you didn't answer the question did you? Why would a touch screen be more efficient or useful than using a keyboard and mouse? It's a very good question. I can think of various instances, but none of them are very compelling, and all seem too much of a niche market to justify Apple producing a line of computers based on touch.
1.) photographers working with lots of images: I can see that using touch in an application like Aperture might be useful. If you're sifting through lots and lots of images it might be very useful to be able to do this by touch. And I can see it being useful to be able to zoom in, crop, and so some kinds of editing using touch.
2.) drawing and painting on the screen are obvious, but this is much less convincing. Without pressure sensitivity and a very precise touch screen the results are going to be so so at best. Artists would need a Wacom tablet anyway.
3.) Older people or people with disabilities that make using a keyboard and mouse painful or impossible. For people with repetitive stress injuries, for example, a touch screen might be a much less painful way to interact with the computer than a mouse. It's hard to imagine an on-screen keyboard being much better than a regular keyboard though, and using your whole arm might not be any less tiring and stressful after the first few minutes.
4.) ??? Especially for the use most people get out of their computers--writing, web, email--there is little real value in a touch screen. You might use it once in a while, but mostly you'd be just using the keyboard and mouse. So what's the point?
As long as there isn't a really compelling use, and with no track history of demand in the marketplace, I'd say this isn't going to happen.
