Naw exceptions to a rule do not make a rule bad. If we let exceptions to rules dictate a rules validity or worth we wouldn’t have rights because people on the fringes abuse their rights to the detriment of society. BMI is good for quickly measuring the health of a group. If the person is abnormally tall, short, or has a very high amount of muscle then they possibly shouldn’t use bmi. For everyone else, it’s a good quick metric to figure out how gross you are. The outcomes for deviating outside the normal BMI are clear, it puts a person at risk for negative health consequences.