Is it possible that the protectors move the area of potential impact down closer to the ball? Unless you change the stand-off or adjust the threading to pull it back up, its still marginally closer to the playfield than factory. I realize its intended to be a sacrificial layer, but more likely to get hit, and if it has the force to break the plastic seems like that same force may have enough to erupt into the 'protected' plastic.
I'm sure its better in a number of ways but wouldn't think they are fool proof.
The washers are ugly but seem to be less likely to transfer energy (as there's less surface area).
That's just my 'thinking' on the whole idea of protectors. I'm not a doctor and I don't play one on TV but I have ridden a donkey and know which side gets the apple.
This would be a good 'pinball mythbusters' theory to try. Jeremy on Tested was on the show and a pinballer, maybe he'll take up the torch.