I originally trained to be an electrician. That said, the argument for ground pin down was if a plug "falls" out due to gravity, the ground prong will be the last pin to break connection. Though honestly this is really unnecessary as if the hot and neutral have broke, what current is there to ground?

Another reason is a lot of manufactures of grounded/polarized wall-warts and GFI plugs tended to orient the damn things for ground pin down making ground up installs prone to falling out. So you wind up with electricians following ground down to accommodate this design flaw (or habit).

I dislike the nema 5-15 blade design as it doesn't hold up well to wear. I'd much prefer a pin and sleeve design with the plug recessed in the socket like CEE 7/Schuko. But that's another argument.