Well, that thread on the forums got some blue attention from Bashiok, who attempted to parry concerns from players. He conceded that it is very likely that Proving Grounds progress will be used by PUG leaders as a litmus test. And, further, he doesn't think that's a terrible idea:
Ensuring a group is composed of competent players (with game knowledge and proper gear) that will not cause a lot of undue stress or a waste of time is not something I would consider elitist. I'd consider that good sense.
If someone isn't great at the game Proving Grounds can actually provide some goals to achieve, in a private setting, so that they can attempt to improve. Knowing that you have room to improve and being able to mark that improvement is a very positive thing for that player, and the future groups they join.
Personally, I agree with Bashiok, with a caveat. I think Proving Grounds will be an awesome evaluation tool for unknown players, for sure. Players can always be carried to achievements, so having evidence that someone cleared a raid really cannot be taken as a definitive guarantee of the quality of a tank, healer, or DPS. Having a ranking that explicitly proves (so to speak) how a player can perform a role in a sterile, solo setting will be a great new metric for recruitment officers or (honest) PUG leaders across the whole game.
I say 'honest' because there are always going to be PUG leaders that will use Proving Grounds grades to find the players who are most likely to carry them to an easy clear. And I worry that an out-sized number of PUG leaders will follow that path, making the game even harder to enjoy for those whose experiences in group content start from a trade chat advertisement.
I think it's safe to say that Proving Grounds will fundamentally change how each and every one of us is evaluated as a player. Every time a new feature has come out that has even passingly allowed someone to assign value or capability to another player, it has been seized upon. But will this new filter will be a net negative to the game, or not? Maybe the added granularity to player evaluations will make for happier groups and happier players as everyone is able to associate with a crystal-clear understanding of the capabilities of their PUG'd raid and group-mates. What do you think?