Curious .... I haven't been able to find anything that directly states not using a vapor barrier.
The 2006 IRC states in Section R702.3.8.1 that “Water-resistant gypsum backing board (Greenboard) shall not be used where there will be direct exposure to water.”
Cement, fiber-cement or glass mat gypsum backers installed in accordance with manufacturers’ recommendations shall be used as backers for wall tile in tub and shower areas and wall panels in shower areas.”
So ... basically this product is an approved backing material to be used in a wet are such as a shower/bath.
The problem I see is if your local code requires the exterior wall to have an approved vapor barrier .... it's unclear if this product qualifies as an approved vapor barrier.
If so then applying this product would require complete and continious coverage from the ceiling to the floor the entire exterior wall in the wet area.