It might not be as pointless as your numbers suggest. If the generator is also a starter (e.g. like a Dynastart) then the potential shaft power could be significant in the event of a fault current flowing. A shorted or low-ohms starter motor can be one hell of a brake. I'd bet that it'd take any pilot by surprise in the event of failure of the turbines to deliver power to the rotor over an urban area at night.
Lemain, it doesnt matter wether the generator brakes like hell or not! There is a freewheeling unit on the engines output shaft ( like your bicycle!! ).
skadi