The exp balancer relies on the fact that your "usefulness to the team" is roughly correlated to your exp, and linear (linear in the sense of super position, not in the sense of a line). It also assumes that the exp is an unbiased estimator of your usefulness. While it's not unbiased, and it's not linear, the negative impact of these deviations decreases with arena size. So in an arena of 20, you would only notice a small difference in the "ideal" balancer, and the exp balancer. In an arena of 100, there would be no difference at all. Since balancing arena sizes of less than 20 isn't a primary concern (we mostly care that big games are balanced), I'm not going to devote hours and hours of dev time to creating the "ideal" balancer, since no one would notice anyway.