I used FreeBSD as my development platform until I got a Mac. The reason why is because the majority of websites are hosted on `nix platforms and as a programmer it's important for me to understand and experience the platform that my code is going to be running on.
If I was a Windows programmer I wouldn't know about cron, symlinks, du, etc... And I might waste an exceptional amount of time trying to code an already available feature in `nix.
I work with a bunch of rxvt terminals, a browser, and a few gvim windows. It's the same damn setup on either Windows or Linux once I have Cygwin and Virtual Dimension (virtual desktops for Windows) installed.
If you're deploying to a Unix server, i.e. not writing desktop apps or systems code, I fail to see how it matters what OS you're using. The dev tools are all ported.
If I was a Windows programmer I wouldn't know about cron, symlinks, du, etc... And I might waste an exceptional amount of time trying to code an already available feature in `nix.