In the population data, there is an effect that relatively thin people arn't healthier than very thin people. This blog discusses this topic in some detail: http://wehrintheworld.blogspot.com/2009/05/bmi-and-life-expectancy-results.html .
It is not hard to understand. People with chronic diseases like alcoholism and Alzheimer's and long-term cancer often lose weight as their various systems fail. So for older people especially, the thinnest people are not the healthiest. Just as important, people who smoke tend to be thinner.
People want believe it is OK to add a pound a year: it actually is unhealthy as you end up fat.
I don't have the answer to weight control, but it is not to ignore it.