Ahhh... still working on the Windows Divorce.
I don't "use" Windows for any real work. However, as a technology consultant and computer technician (I own a computer services company) I do support and work with Windows on my clients' systems and computers. And of course, there is the occasional oddball necessity to use Windows for some one-off reason, which makes the use of Parallels nice. Parallels makes testing Windows easy too, especially things like Windows 7 betas. Oh, and I forgot that I do fire up Boot Camp occasionally to play a Windows-only game. But for any purpose that I make use of a computer to get actual work done for myself, I'm 100% Mac. I don't have the time to waste with Windows.
I think this question would be better if there was some sort of way to define why Mac users use Windows. Is it because they want to (shudder) or because they are forced to in some manner? I know that most of my Windows-using Mac clients do so not because they want to, but because they need Windows for a particular legacy program that doesn't run on the Mac.