I'm disappointed that this mode is clearly meant to be played with a full team of six, and makes no attempt to scale down the robot hordes to accommodate smaller teams, but doesn't lock you out of starting with less than six players either. It's like if Left 4 Dead shipped without teammate AI.
It would have been nice to at least have TFbots work on the defending team. At the moment they just stand around spawn, occasionally tip-toeing to a nearby wall.