I am always vaguely shocked when people use BMI for... well, anything. It was developed around 1830 to 1850 for "social physics" (ie social sciences, ie soft sciences) and NOT, NOT, NOT supposed to be used for anything remotely medical. It's on par scientifically with phrenology, bleeding and humoralism.
The concept "being fat is bad for you" is fine. BMI however is not the way to go about that. I have no bloody idea why people don't just use body fat ratio. It's less complex and actually friggin useful. Calipers are $5, and give you a rough but accurate enough percentage.