Mass balance also helps greatly (on the larger models particularly) by minimizing the forces on the servo gear trains during a hard landing or crash. Without mass balance, some pretty large forces can be transmitted back thru the pushrod to the servo gears, with mass balance, the forces are concentrated in the aileron hinges. This also applies to the elevator as well. Note: if using airdynamic balance, I have found that if you get more than 10% of the area ahead of the hinge line, you may have trouble near neutral, the surface tends to hunt or gallop.