The obvious problem with this argument is that if grid-scale solar installations push down utility pricing, this will kill residential solar and SolarCity's business model revolves around residential.
Residential has zero transportation cost, post-installation, and I think those solar roof tiles SolarCity pre-announced is an attempt to blend the solar installation costs into the roofing costs. "You are paying the roofing guy for labor and materials to build/replace the roof anyway, so why not pay our roofing guy to build/replace the roof, and get solar as an incredible bonus" is I think their marketing strategy with that.