They normally don't plan on healers doubling up with the same Healer.

AST cards overwrite eachother.

WHM lack an aoe dmg mitigation. Like CO or SS.

Generally they don't want healers to be the same Healer. High end content hurts when you double Healers. I would say scholars get it the worst, but low end content like 24 mans and duty finder is really the only place this happens.