I dont see at all how this is appealing in the US.
The subsidy charges are all built into the data plans anyways.
Passing on your upgrades when available, or subsidies up front with a new plan, is simply a bad deal. I absolutely guarantee that in US, you are losing and losing big on that proposition unless we are talking about a prepaid plan.
IMO, few really need the ability to switch at a moments notice, You're better off taking the upgrades as often as you can, even if your intent was to sell the new device.
All I know is that prior to getting an iphone, I was out of contract for 5 years on my old cell phone and I loved it. Just knowing you can leave a carrier if you have problems with them is very satisfying. You are paying for the luxury of taking your phone to any carrier at any time. I would pay more for an iphone to do this rather than subsidize the phone.