I do wonder if THAT many people don't get coverage thru AT&T or t-mobile or a lot of it was just through horror stories they heard. Coverage changes by the day and even though AT&T has stepped up for their network for many years, lots of the old cries from the iPhone days just get recirculated around.
In 2009, I was at a college football game once 2 hours before kickoff and the network already died. I was getting texts 2 hours later even in a mostly empty stadium. Less than a year later during a sellout game against Oregon, I was tethering my Android to my iPod Touch and using an IM app. Meanwhile I could not call my Verizon friends at all because they had no service. Fast forward 2 years to 2012, a few friends and I got separated at the game and all the AT&T users found each other but the Verizon user was totally lost, and it took a while before we finally found her again. Once again, it was clear which service provider could handle the capacity.
This is the Bay Area, so it's not necessarily accurate for all other areas, but what I'm saying is coverage has changed a lot. In fact in a lot of places now if I don't get LTE on my Verizon iPhone, or it somehow drops to 3G, I consider it unusable. I wouldn't call AT&T 3G unusable at all considering it's like 3-4x faster than Verizon's 3G.