I've used spray foam insulation on a number of homes, but never on the exterior. Closed cell spray foam insulation applied on the exterior of a structure can indeed serve as both an air barrier and a secondary drainage plane. It's "structural" contribution is probably minimal. For buildings with masonry cladding, exterior spray foam can help guard against solar-driven moisture. Whether it is appropriate for your needs depends on your climate, the particular cladding of your home (e.g., brick, siding, etc.,) and the size of your wallet. That's a discussion to have with both your designer and builder--preferable at the same time.