Actually there are radios with 14 channels and devices that will effectively double that amount - no, I don't know how they do it.
Really, it's not all that ridiculous when you consider all the stuff that has to be controled on a big heli or a multi-engine plane. Say you have a 4 engine plane; then you have 4 throttles, rudder, elevator, ailerons, retractable landing gear, nose gear stering and that's just the basic stuff. If you start adding smoke, cameras, releases, etc you get to 14 or more pretty quickly. I even saw one plane that required two guys to fly it. One guy would do the actual flying and the other delt with the accessories and ground manuvering.

As far as using your Heli radio for a car, yes, it's likely possable but it's also likely not practicle. A seperate radio is not going to cost that much more than the extra electronics you'll need to buy to use your heli radio in a car.
IMHO, a seperate radio will actually have advantages over your current radio. First it lets you choose between a stick style and a pistol style radio. I use both and for non-air vehicles I find it easier to use the pistol syle. Second, your multi-channel radio is likely a more expensive, complicated radio and if it doesen't have multi-model memory that could make set up an issue. Not too mention the increased possability of damaging the more expensive radio. And finally, if the radio does break, or fail for some reason, both of your vehicles are out of service.