The point wouldn't be to use multitouch as the only way to interact with the computer, but it would be very cool to be able to just reach up and move a window out of the way or something like that.
I keep hearing that, but to make that argument, you assume that it makes sense for Apple to build in a
very expensive screen and write some
very sophisticated software for it (and presumably extend this functionality to the rest of their computers) just so you could occasionally move a window out of the way by fingering it rather than using the mouse.
The cost of multi-touch is such that if they ever implement it in a computer, it'll be a persistent, overwhelmingly useful (and cool!) technology - otherwise, why bother?
iPhone has a touch screen not because it's occasionally more convenient to use it, but because it's the ONLY way to use an iPhone. This solves the problem of having a (relatively) large screen in a (relatively) small device, so it's worth the extra cost. Putting multi-touch in a vertical iMac wouldn't solve any problem while raising the price significantly.