If smoking was going to be banned in cars, I would want it to be for these reasons, in this order:
1-There is a child in the car.
2-It could be a distractor from your driving (I think cell phone use should be illegal while driving)
3 (tied)-"Only you can prevent forest fires"
3 (tied)-Prevention of litter (because adults cannot seem to grasp the concept that the Earth is not your personal trash can)
4-For the sake of convertible drivers everywhere ("this one time...on the highway...")
5-To promote healthier living for society
6-Make my insurance premiums go down
7-If I ever find a used car I really, really want, but someone has smoked in it, I won't buy it and be ticked that someone smoked in my dream car