Holy is the best spec in the game at turning mana into raw healing. This is evidenced by the fact that the first solo heal of H Garrosh 10 man was done by a Holy priest. With enough mana regeneration and proper coordination, they can keep an entire raid up by themselves. However, in a -progression- environment, the best damage is damage avoided (or absorbed) which Disc so obviously excels at. The problem with that particular case clearly being that if a Disc priest does their job correctly, they make other healers either obsolete or severely gimped. -Effective health-, in the current healing paradigm, is king.