That's from a poll posted in Ubuntu Forums. Along with it are some funny options ("because the upgrade is there....waiting") mixed in with the serious ones ("new features").
That's a question I ask myself everytime a new version of Ubuntu comes out. I try to hold off upgrading as long as I can, but then, something breaks with the hardware, or the system is feeling a little sluggish...anything.
Since reinstalling Ubuntu and configuring it to my preferences (I fall within Ubuntu's typical user category), I go ahead and do it.
Been using Ubuntu since 5.04 and I've never had any cause for regret in upgrading. Each new version has brought something better with it.