I lift 3x/week and at 6 feet tall & 160 lbs I would need to put on 35 lbs of muscle to be out of BMI range... That is near my genetic potential for natural body building (~195 lbs@10%).  There is a very very tiny population at that fitness level, and they are pretty fucking easy to pick out in a crowd, let alone by a medical professional.  On top of that BMI underestimates body fat in short people and overestimates in tall people.  So for anyone under 6'3" and not at their genetic potential for body building, BMI is a fine, and even pretty lax scale for what is considered over weight. Disclaimer that BF% is more accurate and should be preferred for individual analysis.