During my one-box pvp forays as a rogue, I found Disc priests to be particularly annoying.
So many tools to reduce damage.

With my RAF, I leveled up a group of 5x priests, as Disc.
They would actually drop just about anything level appropriate on 1x (5x) Smite.
A shaman team, would often need more then 1x (5x) Lightning Bolt.
With 5 of them casting, mana was not an issue leveling up.

The team has been 60th, since before WotLK...
But if I ever decide to level them.
A lot of mitigation abilities on round robin.
A ton of toys like Prayer of Mending on 5x different BG targets.
Massive healing power.
Mana burn would be very painful.
Mass Dispel x5.

I had envisioned them as a change of pace, probably in AV's and towards the back.
As a support role, trying to keep everyone topped up.