I see your point but our manual is not that specific, there is no specific guidence on whether the FMS may be coupled. (simply what nav infomation we may use)
I have witnessed many pilots using the track infomation provided by the FMS whilst following the needles using the heading bug. If they had simply left the aircraft in AUX NAV then their capacity for monitoring the conventional instruments would increase, if the aircraft were to deviate more than 5 degrees then obviously the ADF would be the deciding factor.