I have the same pitman arm nut. The theory is to eliminate side load on the box. Even if you plate the frame the rotational force place between the box and sector saft is still there placing all that force on the bearings inside the box. Any force transferred up you draglink goes through the box still. Plating the frame does nothing to help the box itself. By making the sector shaft in theory rigid to the frame, the load is taken off the box completely.