So there's a bunch of vulnerabilities in older versions of wordpress. There are other reasons to upgrade besides security.

For example, just going from 2.0 to 2.1, I could see a ton of usability features that made my site much easier to manage. And when I read about 2.2 and how a error in your code wouldn't break your site, I wished I had upgraded then. The very next day, I made a coding error in one of my plugins and my site was down the whole day until I could get back to my home machine.

Either way, besides feature upgrades, each version includes better secruity so it's best to keep current.