The flip side is you can choose to extend your rental without giving them any advance notice. And you can cancel your reservation without penalty (at least before these new pay-in-advance deals). Hence they are never quite certiain they will have cars. I have had to wait around for someone to return a car before I could get my rental a few times before.

Also I believe one of them (Hertz or maybe Enterprise) told me if I did the pay-in-advance deal they would guarantee a car was there because they'd bring one in from the airport if needed. Some locations have very few cars and can be hit-or-miss.