Well if you’re pushing the most difficult content then you would want those with the best traits to push that content. If you’re the coach of a track team, then you want those that have the best diet, train the most etc. If you want the best team you take those with the best attributes.
Even with pugs if someone is taking a skill, talent, covenant, etc that is known to not be the best, then it’s more likely they aren’t putting in the effort to be their best.
if transmog wasn’t a thing, would you take a player with a sword thats 100 ilvls lower because it just looks better?