It’s an index. This is what indexes do, they aggregate individual pieces of information to tell you something about a whole. The BMI was never intended to be used as a measure of personal health, but was instead meant to tell us something about entire populations. It’s usefulness on that score remains intact: you can broadly say that, if America’s BMI average is increasing, Americans are getting fatter.
I’d actually go further than this: The BMI probably is a good measure of personal health.
People bitch about the BMI because it doesn’t distinguish between fat and muscle. Athletes and bodybuilders always come up on the ‘overweight’ side of the BMI ratings. NBA man-giant Shaquille O’Neal, for example, is technically obese according to the BMI.
But here’s the thing: You are not Shaquille O’Neal. If you have the lifestyle that 95 percent of the population has — wake up, commute, work, commute, socialize, bed — there is no reason that the BMI doesn’t basically apply to you. The fact that an objective measure of well-being doesn’t apply to professional athletes — people whose bodies are so unique that they make a living using them — doesn’t invalidate the measure.
If you want to claim that the BMI doesn’t apply to you, you have to have a reason (‘I play sports 8 hours a day’ will do). In general, it’s good for societies to set broad, objective guidelines for health. If those guidelines don’t apply in your case, that’s fine, but you need to have a reason, not just a few cases where the index doesn’t apply.