Healers don't really have the dps to farm a few mobs at a time very efficiently. However, the way fanatacism works now for fh is you have a chance to get a stack when you or someone in your party takes dmg. When you aoe farm, you'll be getting hit alot, meaning you'll be getting constant fanata stacks.
Seeing as how chain of light is insta cast for 1 stack, you can almost pretty much spam it. Sure it may only be limited to 4 enemies, but think of it as you're doing 4x the dmg, just spread to your target and 3 other close targets, and since you can spam it, you're attacking 4 enemies at once with decent dmg. That plus cleansing flame on all you pull and fh can actually be fairly decent aoe grinders/farmers. OFC not as good as most other classes.