BTW wouldn't spin rate not help more on 4 seamers than two seamers?
backspin creates "rise" (actually it is just less drop) and for a two seamer you want drop. so spin rate might be more helpfull for a 4 seam pitcher who likes to pitch up in the Zone but not so helpful for a sinkerball pitcher who throws two seamers down in the Zone.
on curves and sliders of course the spin helps too because it increases the downward and lateral break.