BMI is just one consideration of many. I would have thought that obvious.
Percentage of body fat should be part of the overall evaluation, particularly when determining whether or not someone is obese.
The good old Archimedes dunk is still the best way to accurately determine density:
Density = Mass / Volume
I'm sure you realize only a small percentage of the population are extreme athletes with such low body fat composition, which--by the way--has its own unique set of health concerns.