Even though Linux is becoming much more user friendly and widely accepted, most of us still need access to other operating systems. I use Linux about 75% of the time but I still need access to Win 9x for those few applications that I haven't found acceptable alternatives for in Linux. My wife uses MS Office at work and wants to have access to it at home. Other people want to have access to alternative O/S just for fun, training, or to keep current in their job.
I tend to treat new O/S's as you would a game; I actually don't do anything productive with them but installing and learning how to control them is just as challenging and interesting to me as Quake or SimCity. Further, being a Computer Scientist, it keeps me current on evolving technology and has helped me solve a multitude of problems at work. At any rate, for new Linux users, computer professionals, and those just trying to migrate from one operating system to another, I believe using multiple operating systems is the norm rather than the exception.