If you run Windows on a Mac, it's another copy for Microsoft. Better yet, it's on a competitor's hardware. It would be like Microsoft taking it to Apple's home field.
Lifestyle is changing, with greater emphasis on context. It used to be that location defined work and home life. That no longer applies. Even people that work in an office sometimes take work at home.
I think it's too bad in a way. I would have liked to see a new Windows Media version for the Mac, and have Microsoft take the media battle to Apple's home court.
Microsoft has made more of a demarcation of versions around features and capabilities. If you look at Professional and Home, the major difference was networking. Now with the new line-up there is a fair bit of differentiation around features.