While the auto rental Co programs provide very little in the way of comps, etc., they do provide some.

I am up to about 30-35 rentals per year from Budget. A coworker rents about 40-45 times per year. We do get comped up an auto, and can occasionally get an Expedition for the asking (still at the mid-size rate).

This is not to say that there is much in the way of recognition from the auto companies, but there is some. Also, I think that it is National that has different "tiers", much like the airline programs. If you really rent that often; I would bet that National's program would be of use to you. Just a suggestion.