I mean... the robot would decidedly not work fine without joint encoders, on any timescale relevant to the task. You likely could not accomplish one pick/place

I mean... the robot would decidedly not work fine without joint encoders, on any timescale relevant to the task. You likely could not accomplish one pick/place operation. You need the position feedback because you don't have a good enough model of the motors (and everything) to do feedforward control alone. So I'd say position feedback is pretty fundamental to the design. (If you have stepper motors, then okay, you do have a good enough model of the motors.) I think I agree with your point at large, though, that the "proprioceptive" sensors of the robot don't have anything to do with the environment the robot is in, or the task at hand. But now what about an air conditioner with a thermostat? That's got a sensor and an actuator and the sensor in this case really is fundamental to the task at hand.