I wonder how many people actually take the time to connect that grounding spade lug to the wall plate screw?
We had air conditioner extension cords here in the USA with similar plugs. Thankfully they're no longer seen. Unfortunately we still have these adapters to contend with (they work on the same principle):
I have never seen one of these being properly used -- you are supposed to attach the tab (or the lug on the wire) UNDER THE WALL PLATE SCREW. And this only works if the two-pin outlet in question is actually earthed.
And if it is earthed, it's probably easier to just go out and buy a modern 3-pin outlet to replace that two-pin relic with.