You walk up and down the dock looking at moored boats and that's exactly what you see... outboards both in the up and down position!
I have never, ever understood leaving an outboard tilted down while the boat is moored.
Marine growth, electrolysis and cold weather problems are all reduced or eliminated by the simple act of tilting the engine out of the water.
Rather than a directed act to leave the motor in the down position, I think that most folks simply forget to tilt their outboards prior to leaving for the day.
Anyway, I would be interested in hearing others opinions on the subject!