I think it's just an evolution of distro-hopping even though I don't think people like to admit it. After you've tried all of the major linux distros and you're still looking to play around with something different, the next step is to jump to something like BSD or Haiku or Plan9. You might not get a lot of people openly admitting this though, seems like it's looked down upon to try out something new like this "just for fun" versus having some super specific technical reason. Then again, the "just for fun" distro-hoppers may not stick with Plan9 long enough to tell anyone about it before they move on to something else :P