It's a natural fit for AT&T to buy T-Mobile. They both 3G GSM (UMTS) technology. T-Mobile is probably better in some key markets than AT&T, most notably New York (City). The one issue with T-Mobile is it uses the fairly nonstandard 1870 MHz frequency. I don't know of any other carrier that does (anywhere). I assume this is because AT&T has the rights to the more common frequencies in the relevant markets? I wonder wh…
Even if you could simply swap your SIM card to swap providers in the US, the majority of customers would not suddenly become portable; people don't want to pay $500 up front for a phone, even though they're paying more down the line.
As far as I can tell, T-Mobile is (was?) the only major US carrier to even offer the option of buying a phone up front and then getting a month-to-month plan that costs less than the equivalent 2-year contract. That is what I use and all told it is going to save me about $200 over two years vs. a subsidy based plan. But if hypothetically my monthly rate were the same as what it would be on a 2-year contract, as it would be on AT&T, why wouldn't I take advantage of the subsidized phones (apart from lock-in)?
OK, there are prepaid plans, but the choice of phones (especially smartphones) is limited compared to what's available on postpaid.