The problem with the Discipline Priest specialization was that it offered almost no damage output in the early game, and its healing capabilities were practically nonexistent.
The way it healed teammates was by dealing damage—a percentage of the damage inflicted would then be converted into healing. Because of this, no team in their right mind would take on a Discipline Priest during the early stages. They were seen as dead weight.
During the starter phase, most priests who tried the Discipline path quickly switched to Holy or Shadow specializations as soon as they unlocked their first form quest.
