If a ground is run in addition to the metallic raceway, i believe that is critical to ensure that raceway joints are tight. If a coupling comes apart and a fault occurs, the non-continuous raceway enclosing the grounding conductor will act like an inductive choke and limit the fault current flow, possibly to the point of not allowing OCPD operation, or maybe burning up the EGC? Also due to the skin effect, with a EGC installed, probably 90% of any fault current will flow on the raceway anyway. At least so I believe.
