True, it's really for frequent upgraders, but if you do the math it's cheaper to buy the phone outright, or with Edge. If you're not in the middle of a contract, or early in one then do Edge. Edge = ultimately paying the full price, which means they'll give you a much cheaper plan. If you have the money right now, then skip Edge and just pay it full on the spot and still get the cheaper monthly plan that comes when there's no subsidies to cover.
What I don't get is with Edge you get to spread out the cost, but if you're a frequent upgrader then you're constantly running into a situation where you have to pay IN FULL the remaining balance on the phone RIGHT NOW, and that would probably be $300 or so if you're only lasting a year or less with each phone. Someone has money to burn to be able to regularly drop $300+ whenever they get burnt out on a phone and wanna try a different one.
Point being: Edge (spread over 20 months or less) or Point of Sale it's Always cheaper to buy the phone outright. It's just most of us can't drop hundreds on a phone on the spot. So we do the credit option (edge) until we see something else we like, then we go get that and Ooops forgot we have to then PAY IN FULL the remaining balance, so we just then add it to a credit card so we can get the next cool phone....rinse and repeat many times til you have no cards with balances left.....then you're screwed.....