I sort of agree. I travel between 3 cities the majority of the time -- DC, NY, and Chicago. So coverage is the most important in those three cities. I could care less who has coverage in the CA or WA. It not personal, just dont go there enough for that to make a difference in my selection of a carrier. I also travel to Philly and to Miami, but infrequetly, but I can add that as a secondary influencer.
In DC, I would say that TM is good enough - ish. In Chicago, not good enough, and NYC, good enough. Have not looked at the secondary markets because my primary markets already to get a passing grade. I, like you, continue to use AT&T. When the coverage for TM is consistent accross the 3-5 markets where I will actually spend time, then and onl then, will I consider them a viable option.
Still I applaud there efforts to day and wish them well.
In NYC T-Mobile is tied with Verizon for having the best network. They're good in DC as well. Their not very good in Chicago yet as they are pretty starved for spectrum in that market.