He did that in Shadowlands too. I think it's there because he wanted a good reason for his wife to become the spiritual leader of the Frostwolves, which would feel forced if Thrall is supposedly the greatest Shaman ever. I think now it's just their mentality that the major characters are like two classes simultaneously, for example Tyrande being a Priest & Hunter. Thrall is supposed to be a Shaman and a Warrior.