In-Game Parser.

[Felessan]

And I can imagine the amount of rage if the tanks couldn't hold aggro or if the healers let everyone die.
Ohai thar double standard!

You sound like tanks and heals are transparent and dps are not - so dps parsers should be implemented. But it's a lie - all classes have their transparent part, like holding agro, minimal healing for group survival and handling mechanics/killing adds, and non-transparent part like damage mitigation/received damage for tanks, HPS and overheal numbers for healers and actual dps number for dps classes.
I saw as many tank that sucks at proper damage mitigation (some of them even forget completely about their defensive cooldowns) and healers that are very bad at conserving mana and those overheal% are abyssal as possible as crappy dps, and those numbers are not visible anywhere.

I'm refuting that a "basic job rotation" exists in a meaningful sense at endgame, which when you look objectively at the rules of each fight is absolutely true, there isn't one because the mechanics of the fight prevent that.

If you don't understand basic rotation - you'll perform awfull regardless of encounter. Because basic rotation is a first step, further you'll need to know how to restart/continue it after breaks and how to tailor it to specific encounter, but basis stays the same.

The endstate behind the parser is being able to better oneself as a player, not to attain perfection. As such, this answer your second question. People should carry their own weight, not drag down a group and a parser helps tremendously in that department.

But what will happen when some people thinks that their numbers are "ok" as they are not so much dedicated and put limited effort to their improvement and other think that those same numbers are not "ok" because "hardcore guys can pull more"?
This is a main reason for conflict when parser number are clearly visible.
90% of content is casual and thus for people who plays for fun and not for achievement. And this content does not require stellar performance, even mediocre-bad performance are still enough to pass everything but coil and some extremes. People in this content should not be put into the situation when they are actively shun for their performance in content that does not require it, just because someone can do it better. It' bad to force one's playstyle to other people.