I believe his point is, why locked in a prepay rate when you can book the pay at the counter rate and monitor the price, so you can change the booking if price drops a lot, without incurring any penalty as in the prepaid booking.

You have to understand that rental car rates are DYNAMIC - all depends on how the location's inventory picture going into the week of your rental. I have seen rate dropped by 30% going in, as well as running out of cars. It can go both ways but booking a pay at the counter rate gives the flexibility should the rates fall, while guarantee you a car. As long as the rental car companies do not go the direction the hotel chains, i.e. impose a fee on late cancellation, I honestly dont see why people book a prepay rate to lock themselves weeks or even months ahead.