If you place a heavy weight (think of the term heavy as being relative to what you're flying) and you do 3D or extreme aerobatic maneuvers you risk the chance of the weight producing momentum in the tail and carrying it through placing a heavy load on your elevators or rudder. Clear enough???? In other words the plane will be harder to fly "precisely" and take more to recover. Now go back and re-read the above message. That's what he said, just shorter.