Unless of course you want to use it on different networks, ie international.
Carrier phone comes with those fat discounts, so i don't know why anyone would ever PREFER to pay full retail?
Yet people say they prefer to do it.
So that you aren't trapped in a 2 year contract. You re-pay that discount you get over the life of the contract. If more people were not locked into contract and went to the lowest priced carriers it would force overall prices down, contracts keep people trapped and thus put less incentive on the companies to lower prices.