Well, Elon made it very clear on the call:
"We will not sell a house with solar without a Powerwall. That solar can either be solar retrofit, in other words conventional panels put on a roof, or it can be the Tesla solar glass roof. In all cases, it will have the Powerwall 2."
"All installations will have Powerwall. The difficulty of installation will be much less/easier, because the power from the solar roof will only ever go directly into the Powerwall, and the Powerwall will only every go between the [meter] and the main panel. You never need to touch the main electrical/circuit breakers (main panel). Effectively almost every house looks the same electrically, instead of being a unique work of art (needing to re-wire the main panel)."
Elon then goes on to say they are effecictively creating a distributed virtual utility. From a business case, this is quite substantial.