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The PI loop is so you can command Id and Iq to be whatever you want (and they will listen! haha). They are the 2 "dc" components that are responsible for field strength and torque. It's the same thing with brushless DC. But when you have the magnets, you normally want Id to be 0, since there's already a field from the magnets. But when you want to weaken the magnets, you command Id to be negative to fight the magnet strength. For the MGR, you will also command Id and Iq to be various things, but I think the ideal Id and Iq changes as the RPM and load changes. But you still need to command Id and Iq, so even for the MGR, you would go through this process.
I already tuned a brushless DC motor too, and the code for running that motor was basically identical to the code for the induction motor, except for how to know the rotor flux angle. But in each case you still need the PI loop to be able to command Id and Iq.
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