It's sort of like the classic market segmentation problem. Ideally you want to charge each customer the maximum price they're willing to pay, but it's hard to give different prices to different people. Likewise, you want to push each person towards the least harmful version of smoking or non-smoking they'll accept, but you can't tailor general advice like that.

In any case, it seems like much of the e-cig debate comes down to this. Those in favor see it as an alternative to cigarettes for smokers. Those against see it as a way to get non-smokers to consume tobacco.