Oh, I completely wish that Tesla was doing something like the Skyroam Solis. It's a global hotspot that uses virtual SIM card to connect to whatever service provider is available (that they have contracts with, of course) So not only does it use the AT&T network, but it also uses Verizon, T-mobile, US Cellular, and all the little cellular providers that you never cared about until you were in their territory. That would make the car have much greater connectivity.
Also, the car's computer does have a USB port that could have a flash device attached to it. The car could "own" the device, and install an encrypted filesystem for the Tesla ecosystem to safely use for offline storage. As long as the system can handle the eventual device corruption and/or disconnects.... I feel that if Tesla ever wanted us to use that USB port, they would come up with a cable that routed it to the glovebox. USB devices die often enough. Nobody wants to have to tear their dash apart to unplug and replug a usb device ;-}