There are a lot of hormonal issues that arise when bodyfat goes too low. For women, disruptions in the menstrual cycle is a big one. For men, a drop off in testosterone resulting in a lack of other types of performance in addition to athletics.
Not to mention down regulation of thyroid, screwed up leptin etc.
This is maybe the one thing that they got right at the Crossfit Level 1 cert, noting during the nutrition overview that you'll tend to perform best with a little more fat on you than you'd like to see.
No idea where this site got its info, but there's a interesting couple of tables down the page towards the bottom. http://www.sport-fitness-advisor.com...ercentage.html