The problem with a truly "vertical" mouse is the "equal and opposite reaction" principle that forces the mouse to move in the opposite direction to the click unless the thumb is actively restraining it, adding a significant muscle group into the mix and decreasing lateral stability.
A "diagonal" mouse achieves the best of both worlds by giving the wrist a natural angle while allowing fingers to work without requiring the thumb to be actively involved in a "pinching/resisting" scenario.
But there was considerable discussion of that in the other thread on DT.