The study...did not take into account other potential costs of obesity and smoking, such as lost economic productivity or social costs.

There is no question that unhealthy people miss more work; that's a well studied issue. It is also my suspicion that they are probably less productive hour-for-hour than healthy individuals, but that is an anecdotal claim based on my experience with overweight co-workers, and my assumption that when it becomes problematic to move around efficiently, productivity will inevitably suffer.

Furthermore, healthy individuals will pay into the system longer, on average, since they are typically working until, or perhaps well past, retirement age. This needs to be accounted for in any "bookkeeping" exercise on overall health expenses.

The social costs are much harder to measure, but are very real. As someone who works in stroke, I see it up close and personal. Depression is rampant among stroke sufferers, and it appears to be no kind of fun to take care of a loved one after a stroke. Disincentivizing behaviors that lead to increased risk of stroke is a fantastic idea, even if the overall finances are ambiguous.