The real problem is a non-subsidized phone costs you more on most carriers than the subsidized version. Even if bringing your own phone gets you out of having to have a contract, you don't save anything in the process. You pay the same monthly fee as someone that got a "free" phone. So why give that up? It's a tough sell.
The nice thing about non-subsidized phone is that you can use it with any SIM, including prepaid options.
I don't know how the situation is in the US, but in Europe it's quite common and in big parts of the world it's your only option.
For example: When travelling in Thailand I can get a SIM at every 7 / 11 for about 5 bucks with 100 baht credit. This is more then I usually need for the few weeks there and it's extremely convenient when you want to call around the country without being hit by absurd roaming fees.
Having lock-downs of any kind on a phone to me is a killer criteria for not buying it. Subsidized or not.
T-Mobile really seems to have turned their act around. I was with them for a long time but went to AT&T for the 3Gs (at the time being able to tether at a decent speed was worth it).
Now, I don't know. I'm on Verizon and Verizon's coverage was surprisingly good in places I've never had GSM coverage. I'm not sure if I could go back. I wish I could, though - I'd love to give them my support for things like that.
