I keep and carry guns for the remote REMOTE possibility that a criminal will attempt to rob/harm me. Preparing for one remote possibilty while ignoring another is just a foolish and self defeating.
You should have your carry gun on you at all times anyway. Needing a seperate pistol to keep in the car should not have to be necessary. If accessability is a issue, take your carry gun out of the holster and put it in a more accessable place until you arrive where you're going.
If your car gets stolen, you have just given your gun to a criminal. A car is a lot easier to get back than a gun. I'd be willing to bet that as soon as they tossed your car for anything of value, they would find the gun, take it, and ditch the car. You may get your car back, but you'll never see your gun again, and you will have to live with knowing that YOUR gun is now going to be used for crime, and most likely become fuel to the anti's fire.