A flat plate rotor might be easier to make, but I bet it would have some crappy flying characteristics.
Airflow behaviour approaching the stall would probably be fairly 'yucky' (scientific term there).
If that's wrong, I bet all those scientists who have spent hours watching wind tunnels to see how their latest aerofoil designs are working will be kicking themselves for wasting their lives, when they could have just drawn a flat plate on the design sketches and gone down the pub!