This is a bit redundant with post 15, but it's worth clarifying that BMI is NOT a diagnostic tool. IOW, you can't use it as anything more than a screening tool for individuals.
From the CDC website:
BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI. However, to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.
link:
http://www.cdc.gov/nccdphp/dnpa/bmi/adult_BMI/about_adult_BMI.htm
"...but it's worth clarifying that BMI is NOT a diagnostic tool."
Bingo.
The BMI was put together out of height-weight tables that were put together by entities (insurance companies and public heath companies looking for funding) that have financial reasons to find the greatest number of people to be overweight.