That's a compromise. On the one hand you want as little load on the battery as possible, as long as it is not being charged. On the other hand, as you pointed out, it is good to give some visible indication of imminent engine activity.
Make your own compromise: before the first flight of a cold day, you might need every electron the battery has available; but before returning home from a busy fly-in on a hot summer's day, you might prefer to shine every light you have.
FWIW I remember my ground class instructor saying: "There really should be no switch for the rotating beacon, you want it "on" as long as the master switch is."