The major reason for a seperate air only channel is because you can operate ground based vehicles anywhere and there are alot of them out there, if you lose control of a ground based vehicle the results are usually not as dangerous as losing an aircraft. We use an aircraft only frequency to prevent interference from anyone turning on an rc car in their back yard while we have an aircraft in the air. Remember that transmitter signal can effect an airplane up to 3 miles away.
My opinion, if you want to run a ground vehicle use a ground vehicle frequency or at the very least make sure no one is flying an airplane within 3 miles of you.
A friend of mine bought a Futaba 6EXA on 75Mhz for the same price as a 72Mhz for his tank.