Never used one, but with a Trackball mouse you rest your hand on top of the mouse body, and its really mainly your thumb vs your wrist being moved.

I don't know if it's just me, but my thumb does not do fine control that well, and feels quite stiff doing it. It's great for gripping (obviously) and for 'thumping' a spacebar or whatever, but it seems unnatural if I try to drive my trackpad with my thumb, for example.

I see the point about wrist movement though, even on a trackpad there's a lot of lateral wrist movement.