The reason I have multiple "reservations" and only use one, is because it's quite honestly the only way to ensure I might have a chance at one of them having a car for me.

Literally yesterday I showed up at the airport, armed with two reservations for a mid-sized SUV. Neither company had one and both companies were the same parent. I ended up with a smaller car than I "reserved".

I would happily pay a non-refundable deposit at the time of making the "reservation" if that meant when I walked up to the counter, they would have the car I want.