1. I do have to interact with Windows even if I don't use it - and my friends are too cheap to get a Mac and think Linux is a Peanuts character.
2. If Microsoft has good ideas, they will spread. The Windows taskbar turned out to be a great UI device that we see in KDE and Gnome among other places.
3. While I love the Mac OS X, Apple can be somewhat heavy handed with its users sometimes. Want a 17" notebook? No removable battery for you! Not a choice of whether it's important to you, whether you'd prefer the added weight, it just doesn't exist. The greater the competition from Microsoft, the more Apple will bend to users.
4. I can't wait to see the Apple commercials calling Windows 7 Vista 2 ;-).
While some Linux enthusiasts might see a stronger Windows as a barrier to Linux adoption, I guess I'm less hard core about FOSS and would rather pay up for a platform (as I do with my Mac) since I use it so much.