I'm leveling (for tradeskills) 5x Disc priests...

Smite (glyphed, gets +18% Hit), with Atonement (2/2) heals the most hurt friendly target within 15 yards of the DPS target.
With smite spam, they're basically immune to mob damage, so far.

Holy Fire, Smite, Penance... as Holy Damage.
Mindblast, SW: Pain, Devouring Plague, SW: Death... as Shadow Damage.
Smite, Holy Nova, Renew, Penance, Flash Heal... as Healing.

Haven't messed with Divine Barrier yet.
But have PW: Shield (Self) and PW: Shield (Round-Robin) set up.
Mobs (which is totally different from PvP) die well before the shield is gone.

Will have Fear Ward glyphed on all five.
With a Round-Robin, targeting the active toon.
And Mass Dispel, that's my fear protection.
Plus the team can AoE fear as easily as be feared.

Was initially planning (during RAF) on having them as a secondary team.
Stay way back in AV's, and heal/shield/manaburn/mass dispel.