I suppose the kind of "idiots" who don't want data hogs downloading 12gb of data, slowing down the network for everyone else, and only paying $10 to support the network they're putting so much load on. I'm not saying that no one ever has a use for that much data. Some people do. That's fine -- those some people should pay more.
Honestly, I do NOT understand the aversion to pay-by-usage in this country. If you're using a larger share of the pipe than other people, why should you not pay more than other people? It works that way for electricity, water, gas, trash (in a lot of places), even cell service when you're talking about texting or voice. Pretty much every other utility I pay for is pay-by-usage. Why should internet be so different? Local calls on land-line telephones are the one exception, but they are so overpriced to start with, it's not a good example.
Has it occurred to you that if you were charged by the MB instead of by the Mbps, they could uncap Mbps? (at least partially) Food for thought...
I think the problem is that there aren't any good, solid pay-by-usage plans available. There's a catch with the current pay-as-you-go options.