"What does Solar City really bring to the table that they could just get as needed?"
The ability to provide an Apple like end to end ecosystem and the attendent lock-in. Solar hardware and storage optimized for your Tesla. Go ahead, laugh, but people will buy it, just like they buy Monster HDMI cables because the bits are cleaner.
Then again, maybe it's not bad. Imagine a person that would like to go solar and get an EV, but man that crap is expensive. What if Tesla/Solar City could give you a bundle deal? Maybe even amortize your Tesla payments out over the 20 years they estimate for a Solar City installation? Do an iPhone-like upgrade program, maybe every 5 years, you pay a fee and get a new car? Damn it, now I'm starting to get myself interested.