I personally do not have time to smite-heal on my disc priest. Im not sure about others, but I feel like smite has to little healing to keep the tank up if Im the only healer. I actually speced out of Archangel and the smite talents and I'm liking it a lot better. I feel like those talents will be awesome in raids where I have other healers around helping to keep people up.
Then again I'm a scrub, so I am probably doing it wrong.