"I don't think the Standby Power system knows or cares whether the airplane is airborne or not!"
Sorry, but from a Standby Power point of view, it most certainly does. Depending on the model, and customer options, like two batteries for instance, the Standby power switch "Auto" function can be directly related to an Air/Gnd function. That's why I always used to see the avionic guys putting the 737's in Air mode to test the system - every day. Under normal conditions, the Standby Busses are powered from one of the transfer busses (1 I think). If your in the air and lose that bus it will automatically turn on the Inverter and connect the battery to the Standby Bus. Now I freely admit, I'm not familar with the NG, but I can't belive it's all that radically different.
KristianN -It's purely so there is a separation of systems and to enable control of bus switching etc.