Microsoft seems to expect that around half of the people who buy an Xbox 360 will sign up for Xbox Live, despite the lack of interest in the online service.

...he's confident that offering the free "Silver" version of Xbox Live to 360 will ensure that around 50 per cent of purchasers will take their console online. That doesn't mean they'll be playing online, though - consumers will have to pay for a "Gold" subscription for that privilege.
Microsoft executive Robbie Bach also claims that a "new MMO" will be available on Xbox 360 within the next three years, and that the European launch list will be announced in the "very near future."