perhaps you are right, it might be looking at the problem backwards. that would certainly make sense

either way, with the intermediate sheathing you would not have the weathering concerns that you have with exterior sheathing.

there are stick frame houses built with nothing more than rigid foam or cellotex as the sheathing, no plywood. While I would be scared to death to do that for obvious reasons, I wouldn't have any trouble with putting a foam layer as the outer sheathing if you already have a good layer of structural sheathing somewhere else.

The biggest thing to remember here is the cost of the sheathing itself isn't the only consideration. You've also got to keep in mind the cost of installation. This would depend entirely on your local conditions, is it cheaper for you to use a system with a little more labor and save on materials? Or is it better to save on labor and get better (and therefore fewer) materials?

Thanks for the help bmike, it's exactly what I was hoping for. It's best to have someone else's perspective to help you work out all the bugs..