I ran 5 shadow priests way back when and toyed with the idea of 5 holy/shadow, but never did it. The primary thing I wanted out of shadow for this build was mind flay which is the best possible spell you can use on dancing melee (autoface for the win). Obviously with holy you'd have overwhelming healing, sniping ability with smite, cc with chain fears, and mind flay for close up action. Try it out.