First of all, anhedral is ALWAYS DESTABILIZING unless the aircraft is inverted. As I understand it, the anhedral on the horizontal tail was employed due to a flutter problem at high speeds. The anhedral was added to get the tail out of a flow field of a certain flight condition. I'm not sure if this is the actual reason, but believe me the anhedral was not added to make the aircraft more stable laterally. Adding anhedral to the horizontal tail would destabilize the aircraft laterally, which does 2 things: (1) - It makes the aircraft more maneurverable in roll and (2) - it increases dutch roll damping.

My point is that when discussing stability, you must be careful. Adding anhedral increases dutch-roll damping, which can be viewed as increasing yaw stability, but this would come at the cost of decreasing lateral stability. When discussing aircraft stability the two primary modes are longitudinal and lateral. Directional stability (yaw) comes as a distant third. Therefore, I would seriously doubt that someone would try to decrease lateral stability just to get better dutch-roll performance. Yaw dampers work just as well.