Letting a priest get a +dmg belt, letting a druid get a +agility chest, letting a paladin get a dps weapon. Things like this have caused guilds to say it slows raid progress.
But I argue how often have you not been able to start a raid on time because a warlock doesn't have +15 more damage? Or a rogue doesn't have and extra point of crit chance? Or a warrior doesn't have, yet another, shiny 2 hander?
Now ask yourself how often have your raids been delayed due to not having enough healers available?
In this same vein, a paladin offers here why it isn't beneficial to restrict loot by class:
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