Supplementary grounding is permitted per Article 250-54 (99 ref.). You can NOT use the earth as a return, but are allowed to supplement anywhere you feel necessary.
One example of this is parking lot lights. You must carry your equipment ground with you, but if you are in an area with thunderstorms and lightning, you are wise to add a ground rod at each pole base.
Remember the basic theory behind ground rods is equipotential plane, long runs or sensitive equipment your plane can get a bit stretched. Supplementals are not utilized enough, just make certain you are not replacing the equipment ground.
