But it's so cheap! Who cares if my phone doesn't work when I'm traveling. It's not needed then.
And it's the same around here. Not close to NYC or Boston? Forget about using your TMO line.
I switched from AT&T to T-Mobile, and cost wasn't even remotely a concern -- I am reimbursed 100% by my company and can choose whomever I want.
T-Mobile has far superior coverage to AT&T in Miami.
T-Mobile has unlimited international data (yeah, only 2G speeds, but this is all I generally need for Google Maps to locate my hotel / wherever I am going / read my email)
T-Mobile has WiFi Voice which guarantees perfect voice quality in places that both AT&T and Verizon have no coverage due to dense walls (eg: my office).
T-Mobile deploys more data capacity per user than any other carrier in the USA, on average (so you will tend to get better speeds on T-Mobile, as long as there is coverage).
T-Mobile is the only network that supports HD Voice nationwide on it's 3G as well as VoLTE networks.
I haven't traveled domestically since I switched, though I do travel quite a bit, and will of course be critical if it doesn't work for me -- but based on friends who have switched to T-Mobile in the last year, they have found acceptable coverage in similar travel patterns.
As others have said, T-Mobile in the past was obviously a low-cost carrier who's only benefit was the price -- that is simply not even remotely the case today. Yes, their coverage is still less than AT&T and Verizon in rural areas (though it is rapidly improving), but their performance in their covered areas is far superior to AT&T for Data, and far superior to both AT&T and Verizon for Voice quality.