‘Without the FD's you wouldn't know if the automatics are working correctly.’
Assuming that this refers to autopilot (automatics) and the FD, then without assurance that these systems were independent and of equivalent integrity, the use of automation to monitor automation is a very poor practice.
‘Blind checking the blind’.
Without independence or integrity then AP and FD could suffer similar error, e.g common sensor or input failure.
Monitoring must involve independently comparing the autopilot or FD against the aircraft response - the actual flight path, the real world, not as expected or computed.