[smoyer]

People don't like booking the last seat on a jam-packed flight. If given the preference, they'll find a flight with some room (this applies in other cases where people decide whether or not to be in a crowd). So I wouldn't be surprised at all to hear that Southwest's 80% capacity remained constant before and after the six additional seats were added.

EDIT: Here's a related article where church seating on pews versus individual chairs is discussed - and airplane seats are even mentioned: http://www.alban.org/conversation.aspx?id=2380