The posture you'd want when writing on a paper is that of a flat desk that allow your elbow to lean still so that your palm and fingers have full control of the pen.
This is (I only guess) not the case for artist _painting_on real canvas and using broad/long strokes against a canvas, and for that kind of usage I understand what might be the benefits of an angled pen display.
This is (I only guess) not the case for artist _painting_on real canvas and using broad/long strokes against a canvas, and for that kind of usage I understand what might be the benefits of an angled pen display.