I'm happy to have digital copies of games I'm mid-way through completion, and have a hard time understanding why anyone ever complains about free games, especially when it's a new feature added to a price they were already paying, with no detriment.
Halo 3 singleplayer was such a weird, disappointing mess; I am still thinking about going in for multiplayer with this freebie, though I imagine the well-chummed waters are thick with n00b giblets at this point.