Originally Posted by
Tomaski
If someone is actually putting the master switch on without the APU running, then I'm guessing they want to get a leg up on a possible APU start after takeoff (engine failure, generator drops off line, etc) since they won't have to wait for the ram air door to cycle open..
Most modern-ish aircraft work perfectly fine on single generator, bar the ovens and IFE - which isn’t a concern unless you are a certain European airline that likes to fly across Europe on a single engine.