In an attempt to keep this thread purely technical, let me try to explain how I see the vertical undersling's function. I believe the larger that undersling the worse the 1/rev vibration of the system with flapping, since the rotating center of the head moves away from the center of rotation as the undersling gets larger. This means the rotor has a natural 1/rev imbalance as flapping gets large.
The primary reason for undersling I believe is packaging of the several bearings and teetering stops. As the teetering bearings get larger with higher lift forces, the ability to place them along the rotor head axis gets more difficult. The CF forces are passed along a clean loadpath if the teetering bearings are above the path. Otherwise, the bearings cut those beams and potentially call for heavier components as a result. A smooth continuous beam between the blades carries the CF, and the teetering bearings above need only carry the lift forces.