The theorycrafter in me thinks that a dual spec disc/holy priest is the ideal 5 man healer. Holy is insanely easy and powerful AoE heals, powerful tank heals, and decent mitigation. In heroics, i only ever pre cast PW:Shield, renew and PoM, and spam smite... For Fights that have aoe damage, use holy spec. For fights that have a lot of tank damage, go disc and take advantage of all the mitigation.