Better W:L ratio since it better reflects the actual battle effectiveness of the player for the team.
The formula should also contain a factor for shape of the day, last month or so.
And include some formula for classes. Shouldn't be always 50 50 cav on each side, since that would become more monotonous, but perhaps the balancer could pit the archers in one team, and all the cav in the other etc..
Or that pikemen and cav would have a tendency to end in opposite teams or so.
Just ideas, but in general I think there is room for improvement in the autobalancer.