well, look at it this way - if you are in an area with no wireless, you would still have to use the wireless company's network. And don't the phone calls use the network? only thing that uses the wifi is data like IM and web, right?
My iPhone usage is clearly divided on each bill into wireless air time (phone calls) and data. Although both are transmitted on the 3G network, they are quite capable of telling them apart. Same network, different traffic.
It is like your ISP being able to tell the difference between UDP and TCP packets. They are both pushed through the same 'net connection but there are obvious differences.
Similarly, it is possible for the wireless company to accept one and not the other. They could allow my iPhone to continue to receive calls while not allowing data transmission at all.
Oh, and among the things that my phone does that require data:
a) google maps and GPS
c) web access (obviously)
d) streaming media
e) visual voicemail
Most of the apps on my phone require data transmission and without a data connection, it would be nearly useless as my personal assistant.