One reason to use Linux on a Mac for me was to use OpenOffice: the Debian version of OpenOffice is a lot more stable than the OS X version. I'll never, never again put megabytes of text in a proprietary format.

I've also known developers at the company I work for who use Linux for their development work, and OS X when they're home for things like iPhoto.

I personally need a Mac with Linux to test that the KOffice paint application I maintain, Krita, doesn't have endianness issues.

A recent KDE with freetype-rendered fonts just has better font rendering than OS X, too. OS X blurs its fonts something horrible.

Finally, KDE is just a better, more consistent and more capable GUI than OS X. Just take Konqueror and compare it with the finder? No contest at all.