David Morgenstern explores Apple's BootCamp and Parallels along with their deeper meaning in an article at eWeek. He asks "[A]re there really so few native solutions that Mac users must resort to Windows? Or is something else going on?"
He concludes that Apple's making a mistake in promoting BootCamp by sending a mixed message to the market and that "Apple must get out in front of boosting its platform to a wider group of vertical segments."
There are other reasons for using Parallels (and to some extent BootCamp) that merely running software that doesn't work on OS X. Just this week, for example, I wrote about using Parallels to set up test Linux environments for a class I teach that uses Linux--it certainly was a lot easier for me to do that in Parallels than to set aside a separate machine for that purpose. For example, I can have multiple images in different states and easily revert back to a specific set-up.
I also use Windows and Linux running on Parallels to see how Web sites I'm working on look in different browsers and on different platforms. Nothing like having IE running in a window on your OS X box for quick checks.
My students use different VM images on the machines in the lab for running their projects. They're portable and they can easily take the exact environment they're using with them to another machine or even take it home to work on.
I'll grant that this is more about Parallels than BootCamp, but I'm not sure why you'd use BootCamp over Parallels except for the cost ($79.99 for a Parallels license). I'll also grant that my uses are more specialized toward developers than the general public. Still, it's not a small market and it makes OS X a nearly perfect development environment, in my opinion.