Lot of people are dismissing this, but I think there is sound logic to what they're saying for two reasons. One, by becoming a purely focused battery manufacturer, Musk becomes platform agnostic and will have a much better chance of licensing his tech and selling his batteries to all auto manufacturers. This could benefit the broader electric vehicle industry as the technology is now available to dozens of well established manufacturers who can produce vehicles on multiple orders of magnitude greater than Tesla could possibly reach in a decade or more. If they do it right, they could make a huge amount of money this route, dominating a key control point in the electric vehicle.
The second thing is that they will continue to be highly constrained in their manufacturing capabilities for a while. I love Tesla and would love to own one of their vehicles, but the company's production system will take at least a decade or two to even get anywhere close to the order of magnitude that more mainstream auto manufacturers are able to make. This may not matter much if you want your Tesla to be unique, but if your goal is to see the mass deployment of electric vehicles in the near future, organically scaling up Tesla may not necessarily be the way to go.
All that being said, I HIGHLY doubt Musk will go in this direction. His MO at other companies has always been platform oriented with tight vertical integration, and I don't see that changing anytime in the near future, not with him at the helm.