Honestly, I didn't realize that the airplane outlets even had a hole for a ground pin. It's not necessary for safety on an airplane since neither of the two power pins are "grounded," and all of the wall-wart style chargers I've seen don't have a grounding pin, though I used to have a laptop computer that had a power supply with a grounded plug. As far as the weight is concerned, those are fairly heavily insulated 16 gauge wire and can act as a pretty good lever on a flimsy outlet. I think you've just been lucky with the support gained from the third pin.

Anything that increases the length extending out from the socket increases the risk that, even with a reasonably snug fit, it will get knocked out by an inadvertent kick when you or your seat neighbor moves a leg, or crosses your plug when getting out of their seat. Power at the seat is a nice perk when it works, but I've learned not to depend on it being there.

I carry a 6 foot lighter gauge extension cord with me in my laptop bag, in addition to a three-outlet tap with the grounding pin. Now that I've been made aware that the airplane outlets accommodate a three-pin plug, I'll give my triple tap a try next time I encounter a loose outlet. It might help, though I'm going to look for a European-pinned charger, a less haywire solution.

I have an adapter, but (as above) it adds about another inch and a half to the "level" attempting to pry the plug out of the socket.