It's all about image. Microsoft paid money to let Dawnguard come out on XBOX first because it makes them look good. It also has a chance of making money. A 30 day exclusive means people who have Skyrim on both PC and XBOX will be inclined to buy the game twice, making Microsoft more likely to make money.
A 30 day exclusive also means 30 extra days to make a butt load of money.



