I thought the commissioners weren't going to be the back ups for players
It's fairly flexible. The rule is in there so that people don't *rely* on the Commish, or so that we don't have 10 GMs asking us to make their picks for them. In years past, people would just send a PM with a list of names saying "make my pick", without asking to see if the Commish was available. That left the Commissioner, rather than the GM, scrambling to find somebody to make the pick.
Plus, I think the rule applies to the Commish only, and not the Co-Commish.
