On my smartphone, I scroll through the screen with a diagonal motion because I like to see the screen move according to the vertical component of my finger scrolling vector.
I do it, but I think it's more because my thumb rotates around an axis, which causes the most natural movement to be to go from the top right corner to the bottom left. To me, it feels awkward to use the middle joint in my thumb to scroll using a vertical motion, mainly because the motion is much shorter than the radial motion of the joint at the base of my thumb.