You may think that updating your Joomla site to the latest version is not something worth doing every time a new version is released. The site works just fine, and you are not interested in the eventual new additions. You're wrong! There's always a major reason for update to the latest version of Joomla: Security!
Very often security holes are discovered in the Joomla core. These are generally very fast patched by the Joomla security Strike Team, and new versions containing these fixes are released immediately.
If you do not upgrade to the latest version, you are exposing your sites to these threats, and chances are high that you are going to be hacked by someone exploiting one or more of the holes.
As an added bonus, most of the releases contain other bug fixes or new functionality.
There is one more reason to upgrade your site(s) regularly: most of serious, Joomla aware hosting companies, like Rochen, where we are hosted, have in the fair usage policies a clause, which expressly demands from the Joomla site owner to keep his code up to date, else his account can be suspended and ultimately terminated.
The upgrade process, especially if you don't hacked the Joomla site for some reason (as use some of tips listed elsewhere on the site) id generally smooth. Depending on your exact Joomla version can be as simple as uploading a patch package. I normally do is upgrade one of my less critical sites to the latest version and test (always back up your site first!). If everything seems fine I upgrade the rest using various methods, as the Joomla Admin Tools (for Joomla 1.5) or the JoomlaUpdater (for Joomla 1.7) components, or simply overwriting the old Joomla install with the content of the new release.
If you are not confident enough that you can do it, and/or you want also all other components to be regularly updated (highly recommended) you can easily find an Joomla Security Update offering similar to this from a serious Joomla aware company to do is for you.
So the bottom-line is: always upgrade to the latest version of Joomla!