Three prongs was a good idea before the existence of ground fault interrupters, but now that they do exist, ground fault interrupters with two prongs provides a much better solution.
The problem with ground is that it's a conductor. If there's a fault on it, you don't know exactly what it's connected to, so now you have a situation where the metal chassis is connected to something. You touch the chassis and the really-grounded a sink and you have a problem.
Here's a real world example of this. The ground is bonded to neutral at the panel. But what happens when neutral between the pole and the house breaks? This happened to my neighbor. There is still 240V with neutral/ground floating somewhere in the middle. All of his appliances on the less loaded side of the 240V blew up and "ground" was very not at earth ground.
That's a problem with incorrect installation / code. In a proper TN-C-S system the combined PEN will be split up at the main panel, and PE will be local ground and pipework is typically connected to that.
The problem with ground is that it's a conductor. If there's a fault on it, you don't know exactly what it's connected to, so now you have a situation where the metal chassis is connected to something. You touch the chassis and the really-grounded a sink and you have a problem.
Here's a real world example of this. The ground is bonded to neutral at the panel. But what happens when neutral between the pole and the house breaks? This happened to my neighbor. There is still 240V with neutral/ground floating somewhere in the middle. All of his appliances on the less loaded side of the 240V blew up and "ground" was very not at earth ground.