I suppose in real life you don't start your engines with the throttle on "full power" either?
Depends. But throttle wide open, mixture cut off, crank, mixture rich when it starts firing is pretty normal.
Moving some throttles needlessly is a stupid idea because whenever it's moved from closed to open some fuel is pumped.
It would be nice if it could always use the one input device if only a single input device is mapped. For the default config it wouldn't help because buttons would always be mapped by default.