Bandwidth isn't like gas. Nobody's mining extra bandwidth or depleting our nation's bandwidth reserves. The cost of data transfer, for a provider, doesn't scale linearly with customer use like a commodity might; up to a certain capacity, it should cost them more or less (not exactly, but close to) the same to operate a network at peak capacity or one at lower than peak capacity. So if I'm transferring 1 MB/hr or 1000 MB/hour, as long as the network isn't being overtaxed and slowed down by my usage it shouldn't matter how much I'm transferring. This is why a flat rate (or bandwidth tier) makes sense for ISPs, and is a fair arrangement with the consumer.
Pay as you go makes it easier to bilk customers and introduce ridiculous fees out of proportion with the cost of the service. Consider the massive disparity in how much you pay in terms of the actual bandwidth used for text messages, as opposed to voice or data.
Now, I'm not an expert in this. It may be that there's some aspect of this I'm not seeing. But as near as I can tell, pay as you go is strictly bullshit that will lead to consumers paying more for the same service and is a gateway to greater price hikes and obfuscation of actual costs.