A computer is a tool. Use the best tool for the job.
Windows is where I do most of the work that pays my bills (currently; that will change next year with a new project). I like Word and Excel -- there is no free alternative that comes even close to Office 2010 in terms of polish and functionality. And I game (WoW, Dragon Age, old games), .
I judge an OS by how much cussing I do while working with it. Right now, I cuss slightly less at a Kubuntu 11.10 box than I do at Windows 7. The difference is not enough for me to dump Windows.
For software development, I use both Linux systems and Visual Studio. Truly, Visual Studio is an amazing product; however, I'm quite happy and comfortable with multiple consoles, editor windows, and working from the console. Specialization is for insects, as Heinlein said. I'm very Buddhist in my approach to computing and software development; I use what works, be it C, C#, Python, or Javascript. My goal is to accomplish things, not fight some endless philosophical war that has no real meaning.
Sure, I could switch to Libre Office and run games under Wine -- but why? Just to make some political point? That would be stupid. Libre Office is clunky, less capable, and ugly when compared to Word 2010. Most of the stuff I do in Excel doesn't work in OpenOffice or Gnumeric -- lots of complex macros and formulas, DDE connections to live data (that is only provided via DDE)... and again, the "open" alternatives are just plain UGLY.
If I could have only one computer, it would be a Kubuntu or other KDE Linux system. Fortunately, I don't need to limit myself.