less hair = less traction when slipping out of submissions, moisture evaporates faster off a bare scalp cooling you more quickly, no potential for "accidental" hair grabs, shaved hair never gets in your eyes, and even if it's just an ounce, it means less weight to cut. so from a competitive standpoint there are reasons to shave your head.
and then there are fashion reasons, GSP is considered to be pretty GQ with his bald head. And do you think BJ Penn really wants anyone seeing his hairline that clearly?
Everybody good, plenty of slaves for my robot colony?