Chris Zeigler writing at The Verge
Ultimately, it might come down to old-fashioned dollars and sense. If you're picking up a new phone, you'll pay more per month thanks to T-Mobile's monthly payment plan, granted, but other carriers don't offer the inverse (discounting plans for not buying new hardware). At $70 for unlimited data, T-Mobile undercuts Sprint, AT&T, and Verizon considerably, and it doesn't hurt that it's got a brand-new LTE network that it promises to ramp up quickly.
Based on my calculations comparing T-Mobile's new $70 per month plan1 versus Sprint's $80 per month plan2 shows that in order for T-Mobile to save you money you would need to keep your device for thirty-nine months. If you choose to upgrade your device at the normal two year interval any potential for savings goes away.
Ultimately why does it matter that Sprint doesn't offer a discout once you have fufliled the terms of your contract if it takes an additional 14 months to see any savings at T-Mobile.