This thread reflects the confusion that there is, even in the NEC, regarding separate buildings on the same service. Heck, even the IAEI asked three of its' folks, and got three different answers! That's one reason the last two code cycles have had do much re-arranging in section 250.
KISS. Ground rods are for lightning, and only lightning. Each building gets one (or similar electrode).
The green wires are for bonding everything; that is, giving lost electricity a way to find its' way home again- "home" being the service point, where the ground and neutrals are connected.
Since a ground wire is a ground wire, and a neutral is a neutral, the two systems are kept separated everywhere else. You wouldn't want your conduit acting like a "neutral", would you? So, we don't make parallel paths possible.
Since pipe is inadequate for cleaing all but the smallest faults, we run a supplimental ground wire. While this wire is often quite a bit smaller than the feeder wires, it's there.
The effect of all this is to require that sub-panels have four wires run to them: two hots, a neutral, and a ground.
