That too.
As a DPS, I can go to a YT video, or check some online guide, and boom: I got a list of best talents, best gear and, most importantly, best rotation... while for tanks and healers? Nada. There's no "ability rotation" for tanks and healer. It's always situational.
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Making them do more damage then lowers the luster of DPS. In decently sized pulls, sometimes tank can do more damage than the DPS.