I wonder if airlines' top class trainers, the top class XAA Flt Ops Inspectors & technical pilots, the manufacturers top pilots, human factors and human performance experts or a conference relevant to the matter of pilot training have ever debated the merits of monitoring an autopilot flight with FD's OFF. It is a human factors issue more than a technical one. IMHP the FD is a guidance system to achieve a task. Therefore it is more relevant to manual flight than automatic flight. There is an argument that it hinders monitoring if all you do is glance and find it always centred. It can lull one's senses into thinking all is hunky-dory.