Pretty much this ^
And it's not like they can churn out a new dungeon or new boss every week. Did it suck back in vanilla to farm Scholomance 20+ times before getting my Shadowcraft Cap (ninja!) to drop and successfully win the roll? Yes! But it was still fun, and there was always the thrill of thinking wow po "maybe tonight is the night!", rather than just buying it from a vendor.
To be honest, I really don't like currencies in games other than gold.
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