It wouldn't just be abused by people trying to get free Live, it would almost exclusively get used by people doing so. There's tons of people who go forever just getting free AOL months, free XBox live trials and so on. No, Microsoft doesn't want people to sign up without buying a game, because it's the casuals who sign up via the game cards that continue the service. People that hunt down a code online are gonna be the cheapskates doing so in order to cancel after two months.