In an interview with Mobile World Live, Sprint's CEO Dan Hesse says that iPhone users are some of the best cellphone users to have around.
Not only are iPhone customers "more profitable" (in that they just plain spend more money, given the price of the iPhone itself), but Hesse says iPhone users are also more loyal to Sprint (with "a lower level of churn"), and they actually use "less data on average than a high-end 4G Android device." That combination of more money coming in with fewer services going out means that iPhone customers are apparently just as premium as Sprint as the devices those customers use.
That's probably why Hesse spent $15 billion last year to get the phone on his network in the first place. Given how happy he seems to be about the whole situation, it must have been worth it.