A grading scale would be similair to so everyone judging you for your game play. As annoying as it is when dps is low or someone is playing badly it is just a game in the end. And for those wondering about the toxicity of parsers, i personally have been harassed several times while trying to farm for my bird in Zurvan back before stormblood. Even joining learning parties when i knew all the mechs i would still be called out and kicked because i didn't have high enough dps and we would see soar. This is my first mmo. I didnt know what rotation was, i never knew the importance of melded gear. I was just told i suck and that i shouldnt do content because of some numbers. It made me extremely selfconsious about my play style. I would apologize for every death and every mistake and sometimes leave parties apologizing because i knew that i was bad because my numbers weren't high enough. And I would not try the extra content because i was 'a terrible dps' It takes the fun out of a game when you are told you have to hit a certain amount. I never was told how to change so my numbers could improve i was just kicked.
I think numbers and a scoring system would create a gap between players limiting them in what content they can do because the more hardcore players don't want to be carrying a C scoring person in their A scoring team.
It is a game. Its meant be enjoyed by all. Even if they don't play to a certain ideal.