Therefor me Proving Grounds as a tank doesn't teach much of any skill beyond "Pop cooldowns and pray that you get lucky" because pretty much trying Endless, that's what it is, an exercise in how lucky I can be each attempt - I have rotated cooldowns on the heavy hitting wave #6, and still get destroyed before I can get off enough HP to even pop a Shield of the Righteous. It teaches how to deal with packs of adds, which is very little of what a real raiding tank does. Which is also why I would have preferred a tank trial like on Norushen, because THAT is more indicative of what a tank does in raids than picking up groups of adds and getting beat on while waiting for a shitty healer (because that NPC is fucking awful) to heal me and hoping that I don't take a string of bad hits in a row. The number of times I've had to deal with a knockback or some random pack coming from behind are minuscule, and that's pretty much what PG is for a tank.
Personally I think they should have been different for each level, not the same thing with more waves. Bronze is meant for new tanks to show if you can tank dungeons, so you should have more adds and pack management since that's most of what is in a dungeon. Silver and Gold should have focused more on actual bosses.