Well if Xbox live subscription is transferable from Xbox -> 360, and Oblivion does show up on shelfs before your renewal date arrives, I think this is a neat bonus. It helps to somewhat ease the pain of not having easy access to the awesome PC modding community's contributions.
If one is committed to investing in a 360 solely for console exclusives and offline play (not everyone digs online, despite its growing popularity), I honestly doubt this will in anyway change anyone's mind.
I know of people who played Splinter Cell and Ninja Gaiden without Hurrican Packs and Submarine levels, though admittedly they are in the minority demograhpics.
The achievments are obviously a way to make you play games to death and buy more games to accomplish more and have a higher gamer score than your friends so that you spend more money on Xbox360 games... but My God, does it work! *button bashes* 980 Gamerscore! Woot!