The PMC automatically trims fuel flow to adjust N1 to maintain an internally computed fan speed. The PMC only trims the MEC in the decreasing speed direction. The engines do not run rough with PMC failure or Eng Trim selected off, they just respond slower and are susceptible to throttle stagger. With PMC off, the MEC will control fuel flow as a function of power lever angle. Because the PMC trimming authority has been removed, the throttles have to be manually positioned to adjust N1 to respective targets.
The PMC does have an effect on A/THR operation. As tubby pointed out, the TOGA facility cannot be used for the application of power on T/O so engine thrust must be set manually. This is due to the TOGA function of the A/THR system losing its PMC signal. The A/THR will also be inoperative if the digital N1 actual signal is unavailable from the PMC. The A/THR system cannot drive the throttles to a required power setting because it cannot see actual engine speed. In this instance, the MEC reverts to N1 direct with only manual throttle available.