Hmmm. Thought-provoking stuff.

I've used Holy Nova in my PvE group quite often - as you say, it synergises well with other instants (Consecration on my pally, Arane Explosion on the mages, Hellfire on my locks, and even dumping a snake trap down on my hunter).

Perhaps I'll take the obvious cop-out approach - mix shadow and holy in the group, and see which is most effective!