Look at every drug we try to ban, and see how it turned out. Drug prohibition is a disaster. Whenever we try to ban anything, it just leads to a black market. It would be better to have legal tobacco, with it being well regulated and taxed, compared to completely unregulated black market cigarettes and resulting gang violence.
Although I personally don't smoke because of its proven ill-effects, it is still a choice that citizens should be able to make for themselves. There is plenty of efforts made to inform the public of the dangers of smoking. Everything else should be left to the individual. The government has no business telling you what you can and can't put in your body except maybe for drugs that could cause you to do something to someone else such as herione, cocaine, LSD.