Well, if you haven't registered yet, Vista won't be free for you anymore. They just closed registration.
If they give away 10,000 copies to people who are dedicated geeks, who are developing for Windows, maybe doing IT work, or something related to that, who will actually PUSH 'normal' users to switch faster, they will make even more sales for Vista, Vista apps, compatible and upgraded hardware, etc.
Get more people to switch faster = more profit