In the last few years I have rented from every major rental car agency: Hertz, Avis, Enterprise, National/Alamo, etc.
I notice that in every single case when I get there it is a scramble to "find" me a car. Often they do not have the car type I "reserved" and offer me some other kind of car. Also, I notice that on the rental agreement, there is no registration number for the car, it only has the car type ("full size" or whatever).
I gather from all this that when I "reserve" a car, nothing is being reserved at all and all the cars are just in a pool. When a renter gets there they just give them what is in the pool and if the pool lacks the car wanted then the "reservation" is simply ignored.
For example, lets say the agency location only has one full size car left and I arrive with no reservation and ask for a full size car they will give it to me. Now, let's say somebody with a reservation arrives an hour later, his reservation doesn't matter, because I already got the last full size, so he is screwed, even though he had a "reservation".
So, the upshot is: should I just stop bothering to make reservations and show up since it appears to have no benefit to make a reservation?