GM can't wave AH-HA's magic wand and have 76 brand new vehicles (any idiot knows that). As a result, corners have to be cut and less than ideal decisions need to be made. Pontiac does well in Canada, hence a Pursuit. Pontiac dealers need a mini-SUV hence the Torrent. Ideal? No. Purposeful? Yes.
As a minivan owner (Mazda) and friends with lots of GM minivan owners, I can tell you that very few of them would option for the Enclave for 2 reasons - price and functionalaity. They would not, pay the premium that the Enclave will come with. As well, the fancy full length centre floor thingy (brain cramp) looks fantastic, but unless it folds flat into the floor with centre and 3rd row seating to give you a huge load area for those trips to Home Depot then it is not as practical as a minivan.
GM sold over 135,000 of the 4 minivans. You think that they can sell 135,000 Enclaves? They only sold 90,000+ Rainiers, Rendezvous and Terrazas combined.
So assume that Chevy gets a minivan off lambda. It's mandatory. I'd argue that the P-B-G dealers are going to want a van too so as not to lose sales to the chevy guys. Does GM need 2 minivans? An interesting question, but the power of the dealers has an influence. Which brand gets it? I don't know. Maybe the Acadia has more minivan functionality than the Enclave, at a lower price.
The P-B-G think adds a level of complexity to the product mix to avoid product overlap. Chevy can have one of every flavour. Buick can't, GMC can't, Pontiac can't. But the 3 of them combined will. or should.