I guess it depends on what you do and how you do it. Some business dependent software do not exist across platform.

For example, we deploy to Linux servers so we want our dev environments to closely match that, kinda hard to do with Windows. We use Adobe products to visualize our applications, kinda hard to do on Linux. We need to test in IE, need virtualization software if we're running OS X.

All OSes are not equal. Browsers may be, but I don't develop in a browser.