all the metal components of an electrical system should be bonded together, not with separate ground rods, all cabinets, ground pins, boxes, fixtures etc etc get "bonded" which all goes back to source, the bond jumper on the neutral then facilitates a safe point of return for fault current, the earth has resistance and is only to be used as a back up encase the neutral in the service fails. the only time you make another Earth connection (with ground rods or plates) is if you run say a U/G triplex out to a distant building like a barn 300 feet away, at that point due to possible resistance in a reduced ground wire and the extra cost of aground conductor it is acceptable to leave the bod jumper on the neutral and install a ground rod set or ground plate. though recently it is strongly recommended for live stock building to have a separate ground back to source and sized appropriately, due to "tingle voltages" and the difficulty in eliminating them.
