It makes less sense for guilds, but when pugging I can't blame someone for not wanting to basically interview a lesser geared person to make sure they aren't just trying to lie their way into a run.
If you don't spend the time looking into experience/knowledge of classes, the average group that requires 570 will always go smoother than the average group that requires 540.
The average 570 outperforms the average 540. That's a fact. Sure there are many exceptions but I don't blame every pug leader for not wanting to search out those exceptions. Why settle for 540s when you can fill your group with 570s without trouble?
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Higher DPS, capability to ignore many (not all) mechanics, etc. Not a hard concept.
Hell with high enough DPS you can easily get zero MCs on phase 3 normal/flex Garrosh.