but why bother building some marginal build like that just so that you can do a measly amount of damage before the monk turns up and 60 billion damage crits everything in the dungeon?
it's stupid! that's not balanced, or fun, it's daft.
The only way group play works with the current gear levels is Grifts, at least 50-55+ depending on your gear. Anything lower, including torment 10, and you'll destroy everything because the power levels are so high. It's not a class thing either, a well geared wizard can destroy T10 easily near instantly too and it becomes a game of whoever can get to the monsters first to blow them up. Arguably, some classes do it with less gear, but eventually it doesn't really matter. Even Crusaders probably can, even though they tend to be kinda on the slow side for damage even with the new hammer stuff(they tend to hammer a bunch then run away as hammers finish stuff up, while say a monk will just blow everything right away with seven sided strike and move on after).
Once you get to high enough GRs, everyone contribute decently cause stuff stops dying instantly, so monks need several SSS rotations and you can catch up on sustained damage or whatever. But yes some classes are still ahead, and are in general the classes people want in their groups to do high GRs which isn't very fun for other classes. Before it was DH, then near the end of S3 it became wizards and now it's HotA barbs and U6 monks.
Balancing stuff is hard. It doesn't help that they're slow as shit at patching the PTR either, so they'll swing the nerf/buff bats one side or the other, then they'll just try to adjust slightly(generally not enough), and eventually they settle on a build to push live and the balance is off. I guess they have different people working on different classes too and they don't get together to try to fix it all at once or don't consider the only relevant benchmarks for high end performance, GR level, and instead go with average numbers or whatever else and end up messing it up.
That's how you end up with PTR numbers actually being not balanced for the entire thing, and the live situation being exactly the same, even though it was fairly obvious what was going to happen. A lot of stuff could be tweaked simply by changing numbers too, no actual coding involved. Wizards and DH have strong functional sets, all they'd need is a bigger numbers on their set bonuses(like say changing natalya from doing 400%more dmg to 600% more dmg and call it a day or whatever) and they'd be fine, for example. Well for the most part I guess DH is still squishy as shit even with all the dmg reduction stuff. And Crusader will stay garbage tier until they rework the class or add a really OP set I think, class' a disaster.
Anyway point is, if you play in a group and you have good gear, you need to do GRs to have any fun, because T10 is too weak. It's better than if they had left it up to T6 for sure(god it'd be terrible) but still not good enough I feel. Maybe T12 or so would be a decent amount, equal to GR55.