I think getting an assigned seat messes up the priority. I know on the airport standby list (viewable to any employee trying to travel), there is a priority code for Inconvenienced - First and would get a seat assignment ahead of Basic, inconvenienced main cabin, etc. If push came to shove, that priority should get a main cabin seat if that's all that is left. I think by getting the main seat assignment, it defaults back to the status/fare class upgrade ranking.