The Spencer method is the same as Bishop with constant inclination interslice forces.
The Slope methods with inclined interslice forces satisfy overall horizontal, vertical and moment equilibrium, and those with varying inclinations of interslice forces also satisfy horizontal and vertical equilibrium of each slice.
The Morgenstern-Price method is simply a way of deciding where the interslice forces act, and it can be difficult to decide this adequately. We feel that as the same equilibrium conditions are satisfied by the variably inclined interslice force methods in Slope there is little advantage in trying to add these methods. In practice they are found to give answers within about 5% of each other.