Pretty sure people who let their cars run to melt snow are not doing it to melt snow off the hood, roof, etc, they're doing it to let the windshield and rear window defrosters run and heat up the windows. When I brush my car off, I always brush off the driver's door, then start the car, activate the defrosters (and seat heater!) and then clean the rest of the car. Especially helpful if there is ice on the windows, more fun than chipping it off.