Well on the one hand I agree, that is what people do... right now. On the other, I see where Apple is going with this. They're trying to "untether" us from phone numbers. Only then can we move forward technologically. That is, the freedom to communicate seemlessly between any internet-connected device. Think about it... with voip, you can call anyone from any computer, tablet, ipod, whatever. There's really no reason to use the number-based voice system anymore (other than that everyone already does). The concept of a "phone number" is outdated and should be retired. Of course, cell phone companies don't want this to happen. They make a lot of money by locking people into their plans and making it hard to switch networks.
The future is internet IDs (like an apple ID). Take it with you anywhere, use it for all communication (email, voice, facetime, etc) on any device. Apple is once again ahead of the curve and trying to force change a bit early.