Marginal safety increase comes from two places--easier for someone to plug in in the dark (and therefore less likely for them to be feeling around metal prongs they are sticking into hots, since they will orient off the top ground prong), and less likely to have anything fall into the top and shot from hot to neutral. Both unlikely events, both possible.
First you shouldn't be plugging something in while it's dark out. Second I fail to see how putting it up makes it safer than putting it down in this scenario. If I know the orientation, I know how to orient the ground and thus either direction provides the same risk.
What would fall on the top? I have never heard of a single instance of this occurring. Also not all cords have grounds. In fact most lamps do not, so you are only eliminating this possibility on a finite, minuscule, truly measurable unquantifiable circumstance.
A Tin foil cap has a significant social cost and is a straw man. The point is you have two choices, both are as easy, one seems to have a slight safety advantage, at least based on reasoning--although if you have a study on it either way, I'm happy to listen. As to the magnitude the benefit has to have to be worth it, obviously that's a function of how hard it is to install the outlet upside-down instead of right-side up. We have lots of things that confer only a tiny safety advantage that we do when dealing with electric, but we still do them.
You keep using words like marginal and tiny. Those are not terms that indicate true data. Of course it was a strawman. It was used to illustrate the fact that you wouldn't do something when the odds are astronomical. The odds are astronomical therefore nullifying any advantage. It would only be to satisfy an over exaggerated none issue. The odds are just not there to warrant any action that could be excused as a safety reason. It is purely a personal preference and offers no safety advantage, only a perceived one.
You're right, consistent orientation might have a safety advantage--again, we should do a study. But today we don't have consistent orientation, and LOTS of places have them both ways. But if we pick a new consistent orientation, safety might be best served by having it "upside-down."