Im told, if you stack on the spell damage, as holy spec you can be a pretty mean smitebot in instances and in soloing. For me, the aspects of Disc appealed to me more and I never found a reason to change. I had a priest before my Draenei which I tried as Shadow in leveling and I really didnt like it much personally. Disc priests are hard to kill because there is a lot of talents and talent related spell improvements designed to increase your survivability. Because of that, it makes soloing as non-shadow spec pretty nice. Its almost the best of both worlds in that you get some of the dps comparison of shadow and the heal ability of holy.