It depends on the damage the raid is taking during the fight. Most raids notoriously start off with requiring 3 healers, and as the fights get easier to manage, taper down to 2; but then, there’s always that one fight where 3 healers is a thumbs-up regardless of gear upgrades. One good example of this is how ToC-10 demanded 3 healers when it first was released (I think some of the nerfs it took also helped for it widdling down to a 2 healer raid), yet the Twins is always an easier fight with 3 healers… at least, if you have a raid that clumsily throws themselves into orbs like I do. Not like gear is going to fix that, so healing in my smite priest spec is a must for that fight!

I generally look at it like this:

Fight requires three healers? I can smite and heal 50% of the time. Now we’re not losing a DPS spot entirely for that third healer!
Fight requires two healers, not a lot damage going on? I can pew most of the time in smite, but I really should throw Renew on the tank and PoM for the extra Surge of Light procs. NEVER pew 100% of the time, otherwise you might as well be Shadow.
Does the fight have a lot of moving around and strategy, entailing “WTF avoid this damage”? Smite priests REALLY shine here. If you move a lot, not like you’re going to be pewing much in Shadow anyway. In this case, you take up a strong Utility role where you save the butts of people that aren’t doing as hot with Pain Suppression and you try to proc Surge of Light for insta-Smite casts while running around. There are all kinds of ways to sneak in a Surge of Light, including if you went deep enough for Empowered Renew (which most don’t) and through PoM. Good example of this is that whacky Bladestorm the first boss does in ICC.

And bubble for the 3% damage reduction is a must in all three aforementioned situations.