Well, the problem with your argument is that you are looking at how things are right now. We had touchscreen in cell-phones before iPhone. They were pretty ho-hum. And still iPhone was something really, really different. And really, really great.
Yes, we have computers right now that have touchscreens. And they are pretty ho-hum. What makes you think that IF Apple releases an iMac with touchscreen it will be as boring as current touchscreen-machines are? Was the iPhone as ho-hum as the multitude of touchscreen-phones the preceded it?
And what to do with the touchscreen? Well, maybe Apple is moving in to a whole new UI-paradigm? Maybe they feel that the multitouch-tech in the iPhone could be used in general purpose computers as well? From the top of my head, that touchscreen would be very useful in apps like GarageBand, Logic, Reason, iPhoto, Aperture, Cubase and so forth.
Just because touchscreens are a niche right now, does not mean that it NEEDS to be niche, espesially on Macs.