No one is making the point that everyone should be performing at maximum, impeccable play. Using your own terms, a DPS that is doing only 30% of maximum output is bringing the party down and is putting more workload on the other 3/7 members. They play the game too, it isn't just your game, and having others suffer at the expense of underperforming players is not right. Tanks and healers get called out and kicked for not holding aggro / letting people die, DPS should not get a free pass on performing suboptimally. Sure there are those that just don't care and won't listen to feedback, but I'm certain there are more people who would be receptive to said feedback if they were aware of their poor numbers in comparison to others.
As for the argument that parsers potentially turn everyone into raging jackasses who only do so to belittle and kick others, I'll leave this here since it was conveniently ignored: