Didn't one of the devs say at one point that they would never add a parser expressly to prevent the toxicity that seeing others' damage numbers can cause?

As someone who played WoW for the better part of a decade and who's been at both ends of the meters, I don't blame them for that attitude. I've seen plenty of people throw damage meters around either to gatekeep players from "farm" content, justify their own superiority complexes in pugs, or talk down to players who are just trying to get their dailies over with.
Hell, I've even seen that in 14 with streamers using modded parsers, talking behind the backs of players in leveling dungeons.

I understand there is a practical use in being able to gauge one's own damage output, but that won't help you in a vacuum, and to be honest I find the kinds of people who value knowing parses don't often care about using it constructively for others. To open the door for feedback also opens the door for demotivating bile, and given that it is a game, the devs are responsible for making sure we enjoy it.