This question is not to solve a particular problem but rather one of principle to understand the main reason a designer would want to include speed feedback in a motor controlled positioning system. And I'm hoping a veteran motor control expert is able to help answer it.
Consider a cascade controller with displacement being the most outer loop (encoser feedback), followed by a velocity controller (encoder or tach feedback) and the most inner loop a current feedback loop (using shunt resistors, hall devices etc.)
It's clear that current is directly related to torque, and so that helps to overcome torque disturbance as well as speeding the effective electrical time constant. It helps to linearize the transductance of current (command) to torque.
But what does the velocity (speed) loop do to improve performance? Does it further provide linearization? As far as I know there isn't a 'velocity' disturbance to be concerned about.
Typically if you pick the P, I and D gains of the displacement controller in the right proportions (tuning) then you can get by without a speed loop between the position and current controllers.
Cascade control promises tighter control, there are more knobs to turn, but what is the real advantage of having speed in the cascade?