There is a holy trinity but only in expansion raids. Raid bosses are the only mobs in the game that can be traditionally aggroed, as they go after whichever player has the highest toughness stat, meaning a tank can be designated by simply stacking as much toughness as possible. For all other PvE content, it's just DPS and healers, and all healers were AoE healers until EoD. EoD finally introduced a single target healing spec.