This goes back to the basic question of "grounding" vs "bonding". If you do not "bond" your stage to the service "ground" and just drive a ground rod, and say an extension cord gets scraped and shorts to the stage you will never trip a breaker leaving the stage energized and a resulting in a shock hazard.
In my area-and from my understanding of code the proper method would be 4 wires from the service pole to the stage, ground and neutral's kept separate after the "service disconnect" located at the pole and then "re-establishing" a ground at the stage with another grounding electrode. In this case, the "bonding"-separate ground from the pole and the "grounding" serve two distinct but important purposes. In my mind, the ground rod requirement would depend somewhat on specific conditions-dry/wet, distance from the service, etc.