It is possible, actually likely, that some fringe fanatics overstate the value of the BMI (body-mass index), but the most extreme statements I have seen from a fairly reputable source are Michael Fumento's in The Fat of the Land: The Obesity Epidemic and How Overweight Americans Can Help Themselves.
On page 3 he mentions a Norwegian study [H. T. Waaler,"Height, Weight, and Mortality: The Norwegian Experience", Acta Medica Scandinavia 679 (supp.): 1-56] that "found that the lowest death rates for men was below the Body Mass Index (BMI) 25 level, while for women it was a 27 BMI." And he shows how to calculate the BMI on this page.
On pages 8 and 9 he discusses BMI, calipers, underwater weighing, and the newer electircal methods of measuring overweight, as he points out the benefit of the BMI is that you can do it yourself to monitor your weight, the others all need knowledgable assistance (calipers) to expensive equipment.
Page 27 he writes, "In exceptional cases such as bodybuilders, BMI is not an accurate measurement, but for most of us it is accurate, and it is certainly accurate for studying the population in general." I am substantially stronger than average, though definitely no bodybuilder or powerlifter, at 22 before I started really gaining weight, I weighed 170 pounds at 5 feet, 9 or 10 inches tall, almost exactly a BMI of 25. My weight had varied between 160 and 180 pounds over the previous five years. Over the next four years I gained 10 pounds a year, so that by my 26th birthday I weighed 210 pounds. That is the lowest my weight has been since. Over the last 22 years my weight has gotten as high as 250 pounds (thankfully only once and briefly) and twice I've gotten it back down to 210. Mostly I run between 220 and 230 pounds. I got on this personal note because I am more muscular than most people, yet BMI was an accurate measure of my being overweight.
You might notice how many people who attack BMI write that it wouldn't work for Arnold Schwartzenegger. Which is true, but irrelevant. Even people who are substantially stronger than average, but not serious bodybuilders or powerlifters, are not going to have a large enough muscle mass to make it more than slightly off.
And the people who use this excuse are obviously not bodybuilders.
Also, the lower your body fat the healthier you are going to be, so overstating your BMI is not going to be a significant problem anyway. I have just started another attempt to get my body weight down (I'll let you know if it actually works this time) with the intention of getting my BMI as low as I can, I hope to end up getting my weight down to 155 pounds, a BMI of 22. Even if I start getting it down there, I am weight training as part of my effort, and my lean body weight might go up a little. We'll see.
By the way, I don't strongly recommend "Fat of the Land". Fumento too often overstates the case for what he wants to believe. The chapter on fiber is one case, he admits the case for fiber is weak but then says you should eat it anyway because it displaces caloric foods, which is one of the things investigated, and the evidence for this claim is weak. He also dismisses low-carb diets like the Atkins mostly with ad hominem attacks and appeals to authority. But if you really want to know current nutritional understanding this is a good presentation of the mainstream medical position.
(page numbers above are from the hardcover edition)