I've been mulling over Tesla's decision to use AT&T, despite Verizon being a better network in most places. I finally realized that with the ability of the car to use a Verizon tether via WiFi gives the best of both worlds: Verizon superior coverage, if you pony up the dough, and an AT&T fallback, which doubles as a way to transceive when AT&T coverage is better than Verizon (hard to believe, but I have heard of such places existing, mostly the backward parts of the country).
I'll note that this would make an aftermarket exterior Verizon car antenna a possibly smart addition, since it would not fall victim to shielding by the car body. Also, in those rare areas where you prefer AT&T (coverage, cost, equipment failure, bringing the equipment out of range, etc.), you may have to remember to disconnect the car from tether.
All in all, not impossible to handle well enough.