I work for a major wireless carrier and they told us that we wouldn't make profits on the phones we were subsidizing until the last couple months of the 24 month commitment.
Now carriers are putting the cost of the phone in the hands of the consumers and if they upgrade in a year and trade in their old phone, the carrier can turn around and resell that certified pre-owned device and make even more revenue with it.
So, if you can afford it, it's a win win for the consumer and a win win for the carrier because both are making out in the end.
I personally buy my phones out at full retail and so I'm not on a contract, but with this retail installments deal, I can just budget out for 12 months of the installment payments (AT&T Next 12) and trade in my old device and budget out for another 12 months whenever I get my iPhone 6s in a years time. Works out better for those of us who would rather pay full retail for newer equipment more often.