A well designed shooter (IMHO) can outscore most arm designs. It should be able to:
Acquire quickly.
Shoot on the run.
Avoid legal blocking maneuvers.
Place the ball on the overpass.
I know of at least one such arm.
http://www.youtube.com/watch?v=QeQSGmFnKAE
One big problem with arms is the CG. While lifting the 8+ pound ball 6 1/2 feet off the floor, the CG is very high. I saw several arm bots go turtle at the scrimmages.