The cost of the plan, including the EIP is still cheaper than going the old way of buying a phone at a subsidize price and paying a higher rate plan.
The biggest selling point to me is this:
No contracts give you more freedom, instead of waiting 22 months to qualify for a full upgrade price, you can change phones when you want. T-Mobile is creating a trade in program to make this more feasible, so that you don't have to pay off the full remaining cost on your EIP if you decide you want a new phone before you are done paying off your current one.
How many people on here get a new phone every year, hell the apple fanboys are always copping the new iPhone ever year, android fans, some are copping multiple different phones per year. This caters to people like that, that want the freedom and flexibility to keep up with technology and not have to keep renewing contracts.