However, Rob La Guesse makes a valid point.
I guess the key phrase here is Everything in Moderation.
A personal example -- on a really small scale -- follows
I pay fastmail to manage my mailbox with numerous features for a fixed fee a year.
I get a bandwidth allowance tied to the size of my mailbox - at 1GB per month. Since first signing up I got 2GB of spare bandwidth - which I haven't ever needed yet, but I could buy more spare capacity in advance if I ever expect to need more.
This example illustrates what the customer and the provider expect: predictable service in an unpredictable world.
The prepaid spare capacity allows the service to pick up any peak load, and allows the customer to put a cap on any expense.
If additional load would translate directly into additional revenue, you have the time to decide to divert some of that revenue back into more capacity and availability.