That is interesting. I'm not sure why you couldn't just dial a phone number on the touch, and it would do FaceTime only. That would work. But they chose to use Email addresses instead.
Can the iPod Touch not place a FaceTime "call" to a phone number? If it can't, doesn't that mean that iPhone users would have to register an e-mail address with Apple to receive FaceTime calls from iPod Touch users? That seems rather silly, if true.
I get that the Touch does not have a phone number and so iPhones would have to use an e-mail address to initiate a call to a Touch. But I don't get what technical barrier is in place that would prevent a FaceTime call from being initiated from a Touch to an iPhone via phone number (the phone number, just like an e-mail address, is simply an alias mapped to a specific FaceTime device; it's an arbitrary string of characters).