No mate, no it wouldn't. It would be similar to buying a car at 0 down and 0 interest and getting a small amount of gas free for life. But then if you wanted more gas, you had to buy it from the same place that sold you the car. See the difference, there?
Do the math. Actually, don't; I did it two weeks ago when I was looking for a new contract. For a single plan using more than 2GB of data, here's what it will cost you to run an iPhone 5s for two years with all costs including buying and paying off the handset:
At&t: $2640
Tmobile: $2541.12
Sprint: $2280
Verizon: $2040
Hard to argue with numbers, which is what it ultimately comes down to. I'm not saying your 5 person family plan isn't a good deal, I'm saying that Tmobile aren't revolutionising the industry in terms of cost like they claim to be. What they are really doing is presenting what is essentially the same deal as everyone else, in a new format. It's not until you sit down and work out how much it's going to cost at the end of day that you realise that you are still getting screwed.
That is unless you byo your phone like I am doing. Also if you're like my mom and dad who don't upgrade for 4 years paying the "full price" is a terrible deal. Do the math again without having to buy new phones every 2 years and you've got a hell of a better deal. That ends up being 600 over 2 years 750 if they bought a phone to use for 4 years.