Most people who have studied the matter note that Body Mass Index is a deeply flawed statistic. What sense does it make to claim that Kobe Bryant is overweight?
Why is the number so popular? I suspect it’s because it is easy to calculate. Our bureaucrats and number crunchers need statistics with which to do their magic. Hence it is important to them that there be reliable statistics. BMI is easy to calculate, and, therefore, is very useful. What use will be made of this and other like statistics as health care gets even more regulated than it already is, one can only imagine. The more unified our health care system is, the harder it will be for a doctor to be skeptical of the standards created in Washington. The trouble is those standards are often created for convenience of calculation, rather than because of their accuracy.
The abuse of statistics is an old story, of course. Why did "disparate impact" become the standard in race discrimination cases? Because it’s much easier to compare the proportion of each race in a given population with the percentage of each race in a given profession or workplace than it is actually to study each population, workplace, etc. closely. The more things we ask Washington to do, the more likely it is that imperfect statistical models will become the standard for operation.