The Airbus FCOM states that it is not permitted to modify any waypoints from the FAF to the MAPT. So maybe Airbus are taking the legal approach with their advise. I.e. FINAL APP mode should be used during this phase where waypoints are not modified. Prior to the FAF we can make changes to the altitude constraints which allows for some human error to be added to the mix when FINAL APP mode will be following the descent profile based on altitude constraints.