Big companies and organisations often have tightly controlled fixed budgets, and in my experience it's hard to get variable-rate services approved. In fact, even for a personal-use account, I'd rather have a fixed amount than risk that someone abuses my account and I end up with an astronomical bill that precludes food for the rest of the month.

Personally, I like services that cap you to a "degraded" service level once you go over your limit, say you get 20TB at full speed, but after that you're either throttled to 10Mbit for the rest of the month or you pay extra to have the rate limit lifted.

While I agree with some of the mutual benefits of per-use pricing, as a customer I would like some control to set usage restrictions at the upper limit of what I am willing to pay, specifically for services where I don't have much control over the actual usage.