The problem is it creates the wrong incentives. Data is not like water or gas where you can save it by not using it. The fixed costs are the same no matter how much bandwith we use, and any bandwidth we don’t use is lost forever. This means we should encourage people to use more bandwidth, and if we don’t have enough, we should build more infrastructure. Usage based billing encourages us to waste network capacity, and discourages ISPs from building out infrastructure. Why spend money to upgrade the network when you can make money by charging the heavy users instead?