I'm not suggesting anyone remove all the plate glass in their house. We're talking about a pinball machine that's already missing the glass. It's a toy that people often pound on, and the glass is near (or touching) the player's hands. To knowingly install plate glass is lazy, ignorant, and/or cheap. All at the risk of safety.
The difference in price should be fairly small (well worth the expense), tempered glass is much stronger, and IF it does break, it isn't unsafe, nor does it leave a huge gash in the playfield like plate glass is prone to doing. So it's harder (and safe) to clean up... big deal. The safety benefit far outweighs that inconvenience.
"If I need glass replacement I buy plate glass, and if I get around a dealer I'll pick up a couple used tempered sheets." This is exactly what I mean about it being due to being lazy, ignorant, or cheap. Which is your biggest concern, being cheap? Obviously safety is real low on your list. Do you cut off your ground prongs too because you're too lazy/cheap to use a proper adapter or install a $2 power plug?
There's a good reason why pins and videos come with tempered glass, ground prongs, ground braid, and other safety devices. It's not because the manufacturer is trying to spend money where it isn't needed.
Wade