BMI fails to take account of percentage of muscle mass.
Doctors don't use the standard BMI calculation anyway. There is a more complicated version that uses a person's waist measurment as well as their height to calculate their approximate cubic volume and cross reference that against weight and height.
(Seance Trumpethas got no beef with a man enjoying a croissant,
Fri 18 Sep 2009, 22:45,
archived)
I've never had my waist measured at my GP but I have had my BMI calculated.
I know it's not perfect, and certainly not for groups like hardcore athletes or body builders. I expect it's still useful as predictive indicator for things like heart disease.