I don't understand what there is to disagree about?

I would say there is a substantial risk of shock now a days for the old 60s and earlier EMs. Between the coin door wiring of a 60s or earlier machine and the assumption that everything else in the house is wired to ground.

The story from post 4 resonates with me, as I had a WMS Soccer that played perfectly fine. It was next to a High Speed, and I leaned on both at the same time and received a solid shock from lockbar to lockbar. Since then, I have wired the additional ground in for safety when applicable. I could totally picture a child getting wedged between two machines like that... quite disturbing.

The only reasons not to wire the 3rd ground prong and connect to all exposed metal would be to keep the machine 100% original, save a few $, save time? IDK. considering we are usually ok buying repro parts, I don't buy keeping it original. So the only reason to not add it would be to save time/$. Neither of those fly with me if you have the skill to add the safety feature, it seems like a duty to add it... It could save a life from post 4.

When grounds became standard.. Pinball mfgs grounded leg receivers, lock bar receiver, coin door, shooter rod housing, cab sides . Parts of head unit including locks where applicable. At least from what I've seen.

Crazy story. Reminds me of when I decided I was going to mess with my performance ignition system while my vehicle was running.. tiny bit of exposed wire from the distributor was a big zap. I laid down for a while, contemplating if perhaps I would have a special electricity based super power.. but I digress, I had no special powers.. just a sore arm.