Fact: A priest in Shadowform cannot cast Holy spells
Fact: Shadowform is buried a good way down the Shadow 'tree'.
Debatable: A DPSing priest should have most points in Shadow
Debatable: A healing priest should have all points in Discipline/Holy trees.
Debatable: A Holy DPSing priest is not ideal.
Up shot of all this is that Smite doesn't get used all that much, unless you have a very bored healing priest deciding to dps for a while until things start getting interesting. And therefore we get the word 'SMITELOL' or 'LOLSMITE'.
Imagine, if you will, my shadowpriest agreeing to heal for a Shadow Lab run. It's not that bad - I have very decent healing gear and my companions were well geared themselves, for the most part, and very good at the game.
The Ogre boss nearly killed us though. He will periodically make all the party attack each other for 15 seconds, and causes the most outrageous chaos. I failed, in that my four companions all died. I 'won' in that, when the last one died, I started casting 'Smite' on the ogre and praying. I managed not only to successfully cast the spell before the ogre reached me (he would have one shot me), but KILLED him. With a Smite.
Hail Mary Pass, WoW-style