Every Molten Core raid needs at least one Discipline priest for Divine Spirit. This is their spec. This spec typically requires mostly Tier 1 gear to be successful, due to its reliance on crit-ing Flash Heal.

Essentially this build puts 31 in discipline.  Some variations of this build skip Agility and Inner Fire, picking up Martyrdom/Focused Casting and 2 points in Wand Spec. Most skip Force of Will for Improved Mana Burn and Inner Focus.  The holy tree used typically picks up Improved Renew and Holy Spec, then Spiritual Healing and Subtlety.

This build gives the strongest mana regeneration capability while strongly improving the healing tree.  This build also gives better Player versus Player capability than either Holy build due to Improved Mana Burn/Inner Fire.

The disadvantages of this build are that priest does not heal as well as holy based specs and the priest cannot solo well.