Sure, BMI is just a statistical tool, but someone 6 feet tall with a BMI of 48 is going to weigh 350 lb. Ignoring BMI for a second, someone (that's not a powerlifter) at 6' and 350 lb is going to have a high body fat percentage, which also correlates to a number of health concerns.
Anyone is entitled to disregard BMI, and I agree it's not a particularly useful number. However, if your body fat percentage is high the "average person" is still at risk. OP, it sounds like you're claiming you're a corner case, but that doesn't invalidate metrics like BMI and body fat percentage for people that aren't you.