Yes, forgetting about the fact that it's a pain in the ass to get out of Xbox Live membership. I guess they will only accept creditcard or your bank account, so you got to enter those things somewhere. Then it also will be a pain in the ass to sell the 360 during that duration. I mean the "membership" will be forced on you. If you pay it "fully" on credit, you can sell the 360 afterwards w/o issues and pay back the credit using the monies you got from selling it. But using this scheme, you are forced into a Xbox Live membership. You also got a forced warranty that is probably not sellable at all. And you got Xbox Live membership, also not sellable at all. And in case you don't like Xbox Live at all, well crap - you got to pay for 2 years anyway. And maybe they even lock the 360 onto your account, so it won't be sellable at all.
And selling it isn't that off. I mean if you can't afford the full price of a 360, you should better not get it at all or at least get it in a way, that you can get rid of it w/o problems, if you need the money.