BMI measurements (based solely on weight/height) are poor indicators of health. BMI doesn't take into consideration fat/muscle, etc.
A recent study suggested that about half of overweight people and nearly a third of obese people have normal blood pressure and cholesterol levels, while about a quarter of people considered to be normal weight suffer from the ills associated with obesity.
The assumption is that thinner people must be healthy and must engage in healthier habits. After all, they're thin! It is impossible to tell from someone's BMI alone whether people are healthy or not, what they eat, how much they exercise, etc.
True, but the BMI is a simple number to get. And according to the article someone who is 5.6 and weighs 220 has a BMI of 30. So a BMI of 35 is way up there.