You are extremely wrong on how you view shadow priests and how to utilize their utility or where to place them. In vanilla (not classic) I was a raiding shadow priest who cleared Naxx several times with my guild back then. I continued to raid as shadow all throughout BC as well. Shadow priests were one of the top tier dps classes in BC and we weren't put in healer groups in BC because there was a shaman in the healer group for mana spring totem. Shadow priests were put in the warlock and/or mage group to provide mana to them and if just warlocks were in that group it was so they could get constant health regen from VE. VE allowed warlocks to lifetap and get healed without having your healers waste mana to throw a hot on them or heal them directly if they didn't take damage from something else. Quite frankly, the synergy that warlocks and shadow priests brought to the table made mages get sat out because mages weren't as good as shadow priests + warlocks in your raid. Mages brought no utility in BC other than conjuring food and giving an int buff. Only one was needed in your entire raid to provide those. Shadow priests and warlocks brought multiple utility spells and having mulitples of each didn't minimize the utility the other ones brought (2 shadow priests could give 2 different groups the benefits of VE & VT and warlocks could keep improved SB up nearly constant with multiple ones in a group & 2 warlocks meant both curse of elements and curse of shadows for more casters getting more utility).
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So was I! In my guild I would routinely compete with the other DPS classes for the top spot on most fights. Sunwell was like our moment to shine because I don't think there was a fight I wasn't top dps in there. And when it came to arena once Sunwell hit, we were gods!
Very well said and very correct. But I think you're forgetting that warriors also wore leather in quite a few slots too because most leather had strength on it not agility. In fact, if you look at a BIS list for warrior after Sunwell hit, there were still pieces in there that were leather.