Regardless of the ill effects of smoking, my argument will be that each citizen has sovereignty over his/her own body. This includes the right to choose which substances to put into your body. The choices you make might do harm to your body, but that doesn't mean you shouldn't be free to accept the risk. It's your responsibility to deal with the consequences of the actions you choose, not the government's responsibility to protect you making those choices in the first place.
So, to reiterate, the government should not prohibit us from making lifestyle decisions for ourselves, even when those decisions may prove harmful to us. It's better for the government to stay out of such things and let us assess the risks for ourselves. In addition, case after case shows that top-down bans of particular products do not stop people from consuming those products, so a smoking ban would likely be ineffective as well.