The point is to let the price fall faster after launch. Xbox can't exactly promise "Our games' prices will fall faster than the other guys" because the market will determine that -- in aggregate they will, but there's a reason Skyrim is still $30 on Steam.
Microsoft wants a world where you have account-locked games tied to disks, because it gives them the benefits of Steam-like DRM but also the game conveniently on a disk to take to a friend's so he doesn't have to download it, and to let the poor schmuck in rural Nebraska with 25 kb/s to play the game some time in the next week.
Honestly, Sony can take the same pricing strategies as Microsoft on their digital downloads -- Microsoft just wants to have more convenient digital downloads, while Sony just wants more convenient disks.
My guess is that long-term Microsoft will prove to be on the right side of history (the success of Steam is evidence). Sony can probably do just fine for now, they can always transition in later generations when the downsides are no longer breaking news stories.