In a blog post-cum-interview about obesity and health savings, Megan McArdle writes:

With health care in the news, everyone's looking for magic bullets to save money.  Obesity seems to be a growing favorite:  wouldn't it be great if we could make everyone look like Jennifer Anniston, and be cheaper to treat?  There are a lot of holes in this theory--the morbidly obese are very sick, but die young, while lower levels of overweight/obesity aren't so well correlated with poor health. But still, the idea's power seems to be growing every day.

As someone who feels totally fine slapping additional taxes on soda or cigarettes -- in part to reduce public health consequences like obesity and lung cancer -- let me say that I don't think the best justification for this policy has a whole lot to do with to do with reducing health spending. A less obese population that doesn't die young from fast-onset lung cancer might end up spending more on health care. Totally possible.

And that's fine. Taxes on public health threats will still raise revenue. And they will still reduce the amount of behavior that damages public health. The value of reduced obesity and lung cancer rates shouldn't stand or fall depending on whether or not it saves society money. They are valuable because our society considers morbid obesity and early, horrible deaths bad things that are worth discouraging. (Our society might also think that universal health insurance is a good thing that requires additional public revenue, but the tea leaves on that seem somewhat unclear at the moment.)