I just bought a Rossi .357 for a car gun- conveniently enough, the guy who sold it to me gave me a left handed Uncle Mikes holster for it. I'm planning on mounting it under the steering column somewhere for easy access while in the car.
As for why keep a car gun, for me, its like any other BUG, serving multiple purposes:
1) in case of firearm failure
2) fastest form of reload
3) arm someone else in an emergency