Well most of the time you don't have the ability to field too many armies with 20+ Devil/FoD. I'd prefer to have 3 or 4 Armies hammering away from different directions and each having the ability to null enemy mages than one (or 2) Big ugly armies that can be shut down by a Storm.

Just a personal preference but this is from a MP aspect. The AI rarely uses storms or storm items to their full effect.