People always say this, but in the United States, T-Mobile is the only carrier that even has any alternate options. Everyone else doesn't discount your rate; you pay the same whether you buy a phone outright or buy on subsidy. Buying outright simply prevents a two-year contract (where the early termination fee is usually cheaper than the sticker price for the phone). There's no reason not to buy without subsidy unless you use T-Mobile.
T-Mobile offers a couple of different options, one of which is a $10-$20 discount on the monthly plan for non-contract phones, or a zero-interest amortized 24-month payment plan that allows you to still get the subsidy and the discounted plan. To me, not having to drop $500 to get a phone when I don't have $500 is worth $10-$20/mo (which, over two years, depending on the phone you get and the price you pay with contract, is usually pretty close to the sticker price for the phone anyway, maybe +$100-$200 than buying outright, a reasonable finance charge), and I didn't know about the payment plan when I signed up, though it's generally been agreed that that turns out to be the best option.
T-Mobile offers a couple of different options, one of which is a $10-$20 discount on the monthly plan for non-contract phones, or a zero-interest amortized 24-month payment plan that allows you to still get the subsidy and the discounted plan. To me, not having to drop $500 to get a phone when I don't have $500 is worth $10-$20/mo (which, over two years, depending on the phone you get and the price you pay with contract, is usually pretty close to the sticker price for the phone anyway, maybe +$100-$200 than buying outright, a reasonable finance charge), and I didn't know about the payment plan when I signed up, though it's generally been agreed that that turns out to be the best option.