I've been pondering this, too. Trying to think logically, what best engineering practice would be. If bonded/ground properly, either way would be safe for people. Driving a ground rod at each outbuilding would be better for lightning protection, though, as the bolt would be driven to ground with less impedance. For a garage, this is appreciable, but not so much for a shed surrounded by taller buildings and trees. For both a garage and a shed, the new local ground rod would be a better design, but I'd consider both to be acceptable unless there was substantial risk of a lightning strike.
As to your quote, it appears to me that 250.50 only refers to all grounding electrodes that are present at each building or structure served be bonded together. It does not imply that a grounding electrode must be present at the structure, just that if multiple ones are used, they need to be connected.
[This message has been edited by SteveFehr (edited 11-29-2006).]
