Heretics are a hybrid. They can be casters, tanks or healers. Normally when I'm running in PUG's I'm either the main healer or the back up healer. I do field base buffs. I do massive spread heals constantly to keep the group alive. I also protect our casters and tank when I'm being attacked. It's not easy to kill a group with a healing heretic.
On the other hand heretics can be brutal damage dealers. Their damage spells can get to unreal amounts if they have the time to get that high. This doesnt happen much in RvR. They can be decent tanks if they know what they are doing.