Okay, I ride with my bike on the car often. Here's when I do it...

Monday mornings, bike on car and into work. Car stays in the company garage with my dress clothes in it. Friday afternoons, back home (stopping at dry cleaners)

Occasionally on the weekends. When there is no wind, i prefer to drive to the lake rather than ride. On stagnant days, there are CLOUDS of gnats on the route to and from. I hate gnats. Why put myself through that when i have a perfectly good car.