I think this is all marketing. I've been with AT&T for years and I get the new iPhone every year and sell the old one, with no intention of switching carriers.
Seems like if you are happy with your carrier, there really is no benefit to being contract-free since I don't have to pay a monthly installment for my iPhone on top of my plan cost every month. Kudos to T-Mobile for trying something new, but the amount of money you have to spend to own an iPhone ends up being about the same.
Plus, T-Mobile has no LTE in Boston, so that would be a deal breaker right off the bat.