Tesla have shown a rather large appetite for vertical integration. For example, they make their own seats. Why? I guess they think they can eek out some advantage. They could certainly get commodity seats from a traditional auto supplier, but they must have some line-of-sight to making them cheaper, or better or maybe just more customized than what a supplier would produce.

I think the same will be true for batteries. They won't be making a copy of the Panasonic cell design. They will only become a manufacturer if there is some advantage to be gained. They will probably seek to integrate IP from their Maxwell acquisition and improvements from their research group in Canada.