Most of the time people whining about BMI are fat people...
BMI is a poor measure of absolute overall health, but as a basic metric (to be used in conjunction with other measurements, or common sense) it is fine. Its also extremely simple to calculate, unlike say body fat percentage. Obviously body fat percentage is far more valuable, but it can be time consuming and costly to accurately measure.
Fatness correlates well with ill health and BMI is (most of the time), going to provide, a reasonable level of your fatness because most people don't have the muscle mass to make BMI inaccurate.
Obviously health premiums solely linked to BMI are stupid, since you can easily have a overweight/underweight BMI and be far healthier than a normal BMI person.