"Unlimited" plans just hope that not too many people use a lot of data. But if you're really one of the people who doesn't use much data, you might as well use

"Unlimited" plans just hope that not too many people use a lot of data. But if you're really one of the people who doesn't use much data, you might as well use a plan which charges per gigabyte, and then you're exposed to the real per-gigabyte cost. Market forces therefore keep the cost of unlimited plans roughly in line, and the per-gigabyte cost is still relevant even for unlimited plans.