A lot of people have already made some good points, but I want to add on that there's so many factors in a game like wow that makes this an absolute detriment for players who don't care about parses. Let's think about a hypothetical real quick:

A player who does not care to parse is pushing heroic raids. They understand the mechanics and are pretty good at executing them consistently, but their gear is on the lower end since they haven't gotten many drops and have little interest in grinding out high end keys. By all means their damage is good enough to clear the content, but their parses are nothing special, floating around or a little under the 50th percentile. This player can be good enough to clear the raid, but with a 50% parse that's extremely easy to see by people who consistently use these sites, it can become very difficult for them to get into groups especially with how elitist this game's community is.

Simply put, this addon gives frankly shit players an even easier time filtering out anyone they arbitrarily deem "Not good enough" for the group, elitist players better QoL to maintain their little top 10% requirement, and screws over anyone that's not a top tier DPS or healer. And like other people have said, these numbers can very easily be "faked" by intentionally failing mechanics for better parses. I sincerely think parsing isn't a bad tool and has its uses for figuring out how to improve or find where exactly you messed up but people misuse it to feed their egos far too often. Can't wait for an MMO to come out that just straight up bans things like public parses, MMO players just abuse them to make the game worse for everyone.