Before I took up multiboxing my main was a moonkin.

I think you should keep one moonkin out of the healing rotation. With four of five party members exclusively casting, you're gonna see a good return on that. That one moonkin would be doing more damage than the rest of them, so in the event that the tank loses aggro, the mob is going straight for the Moonkin with his 400% armor instead of the other balance druids. The moonkin has just a bit more survivability, and thus gives your tank more opportunity to take back the mob.

ALSO: I would love to see a set of moonkins macroed to cast Force of Nature x 4 simultaneously. In fact, in PVP that would be pretty funny to pull out on any class that can't crowd control 12 mobs at once.