Why? I'll tell you why
You seem to be forgetting that the "iPhone" also includes the iPod Touch.
Then with a $20 mike-earbud combination I can use my iPod to make phone calls.
Since I have a non-AT&T cell phone that's only medium interesting, but I can imagine a day perhaps when wireless is ubiquitous enough that I could dispense with the cell phone entirely.
Paris, because she's in a lot of people's dreams too.