There are several good reasons to have two operating systems on your computer:
- Learning opportunity: linux is a cheap and easy way to learn the world of UNIX, which is not going to go away despite what some people in Redmond may believe.
- Security. It's not just Firefox; the entire Linux platform is generally more secure than Windows, and it's much more difficult to write viruses that will successfully propagate across Linux platforms (the non-homogeneous nature of Linux and the inconsistent layout of filesystems, locations of binaries, etc between distros doesn't help virus writers either).
- Troubleshooting. There have been many occasions on which I've determined that an apparent hardware problem with my PC was actually a driver problem by taking note of the fact that it worked OK when I rebooted to Linux.
- Website development. Generally speaking, you're better off with a UNIX-based webhost if you want to have a phpBB forum, and you'll be more adept at using the resources of that webhost if you know Linux. I've lost track of the number of times people who obviously had no UNIX knowledge asked for help doing something with their board (such as scheduled backups that are automatically transferred to a remote site) that would be trivially easy for a UNIX user but confusing and opaque for a Windows-only user (crontab? ssh? What's that?)
Maybe somebody else can think of more reasons, but those are the ones that come to mind immediately for me.
PS. I use Mandrake. And yes, despite widespread claims that it's only suitable for beginners, I like it despite the fact that I have several years working experience as a Sunsoft/SOLARIS and SGI IRIX sysadmin. I've been around the UNIX block; I've used numerous kinds of UNIX including the really big and expensive ones, and I see no reason for the contempt in which some people hold Mandrake.