I understand the desire of video game developers/publishers to let the players beta test your new content and save money, but PTRs have become so much a part of the development process that nobody has stopped to say "why are we doing this at all?"
After all, if the entire point of playing a game is to experience new material, why allow everybody the ability to cheat like this? Is it just so that the elite raid teams can get "practice time" in on the next raid, and then they can provide a strat for everybody else?
If that's the case, then the entire point of MMOs is simply "how quickly can you finish a raid".
As you can tell, I dislike the concept of "open betas" and "PTRs" in the first place. They keep all players from experiencing content at once, and instead of everybody muddling through and trying to figure out how to handle new content, the sandbox is already there for people to try to figure it out beforehand.
I was in a run --I think it was Upper Blackrock Spire-- when one of the players commented that it is so much better now in Classic than in Vanilla because we all now know what to do; back then we were all muddling through trying to figure it out, but now we've the advantage of 15+ years of insight to know how to handle things. While I agree that it is nice to have detailed data to fall back on, I'm not convinced it is "better" now. We just know what we have to do, and all we're left with is just execution. There's no "hey, this isn't working, let's try this instead" or "WTF was THAT attack about?"
The thrill of discovery, and the knowledge sharing that happens following a "eureka!" moment is all lost. Now it's all about "Go look it up on the forums" or "Check out the YouTube video for how to run it".
Perhaps if dev staffs got back to not using PTR servers, MMOs might see a resurgence in that Vanilla excitement that nobody seems to be able to generate anymore, yet everybody complains about.