There was a discussion about this on "another website" recently. I think it boils down to if the motor is the wrong type (which most buffer motors are) it wont go more slowly, but you'll lose all your torque, so very little pressure on the wheel will slow and stop it. If you still have plenty of torque with the motor slowed you should be ok.
Went and double checked it. If its a brushless or synchronous motor it shouldn't work, at least not with a normal rheostat that just reduces power. I have no idea if that light dimmer switch is different somehow, though.