I disagree completely. It's just like real estate: since I only use about 50% of the floor space in my apartment at any given time, I roll over my unused square footage and only pay rent every other month.
The difference is when you don't use your bandwidth, the company benefits, but when you don't use your square footage, a company doesn't gain anything.
The ISP company only benefits if their pipe is saturated 100% at that moment. If they had spare capacity at that point in time, it's of no use to them and they can't carry it over.
