Problem with that is you're relying on the surface properties of the material you're pushing against. If it's too soft, you get no motion. If it's too hard you

Problem with that is you're relying on the surface properties of the material you're pushing against. If it's too soft, you get no motion. If it's too hard you get too much. The nice thing about using internal reaction wheels is that the torque you get is entirely independent of the surface you're resting on.