For security reasons it is absolutely critical to keep software up to date (even Apple are allowing auto-update in OS X Mountain Lion). WordPress is no different; with each new version of WordPress comes new functionality however security updates are also included in WordPress updates. There are over 50 million Websites powered by WordPress. The chart shows the versions of WordPress powering self-hosted (wordpress.org) Websites.
It really amazes me that just over 1 in 4 WordPress Sites are running the latest version of WordPress. Seriously, for security reasons, update your WordPress software, Themes and Plugins.