If the loan is 0% interest, which this is, it's better to pay over time and not all up front if you have good fiscal responsibility. I only buy things like this (not a house or car) if I can purchase it outright but I'd rather pay over time so I keep the money earning interest (what little it is) or sitting in a money market account or mutual fund. It's great to not have monthly payments but if monthly payments have a 0% interest rate, why not pay over time?
Plus, some people would rather not deal with selling the phone. I don't mind so that's the route I go but it's not worth the time or hassle for some people.