Okay, the title is a little dramatic. But you have to imagine the pain that product managers at 1000's of small & medium companies are going through: testing your product on four versions of 'one' operating system - and that is before the service pack. Home Basic, Home Premium, Business and Ultimate!
Sure its one OS, but they do have different capabilities (especially for graphics) and you can't leave this to chance. Plus the drivers issue that's been plaguing lots of users creates an additional complexity - "hmm, our product works on our machines, but will it work on the users machine?" . Users, both consumers & business/enterprise customers, are likely to buy systems with any of these four. That means IT and testing have to have provide access to all four of them (with or without SP?) to all/most of the developers. And testing teams need full & dedicated access too. Fortunately VM's have made this task a lot easier.