Do humans grow in 3 dimensions? Research suggests the power should be slightly less than 2. e.g. http://www.ncbi.nlm.nih.gov/pubmed/15761809

Some groups of athletes list overweight because muscle is very dense - BMI doesn't distinguish between muscle and fat.

However, this does not mean that BMI is useless even here - overweight athletes often suffer similar medical conditions as overweight people generally.