Making something law and enforcing it is 2 different things. Non-hands-free mobile phone use in a moving car is already illegal.
I don't often side with the smokers as it is a filthy habit and deserves the disapproval it is finally, at long last, getting: but it seems to me that if you are in your own car, on your own, then it is probably a step too far in protecting those unable to protect themselves, to ban smoking.
Now banning it whilst on the move as it is a distraction, that could be up for debate.