The issue is a financing decision, because you pay the full cost over the life of your contract if not (~$20 per month higher cost). Hasn't always been that way, but T-Mobile and earnings pressure has everyone moving to separate the subsidy from the service. I'd rather get the unbranded phone and be free from carrier interference that I can use how I want on any carrier, including when I go abroad. Its stupid small things that make a huge difference, whether its higher internal memory, wireless charging or being able to set MMS apns (you can't on Verizon phones, the setting is hidden in the registry which you can't modify).
I completely understand people wanting to get a carrier phone, but the 1520 with less internal storage, no wireless charging and carrier locked costs more right now than the unlocked version, with the only difference being warranty. I know I get something like a one year warranty when I buy something with my Amex, so I'm not too concerned, but that works for my situation and not everyone's.
We are probably 5 years away from a truly European style market, but I think we'll see that once Verizon and Sprint move away from their 3g networks. For now, though, there is enough pricing pressure on the carriers to make unlocked phones pretty compelling