Every new release of Joomla! comes with the developers' advice to update to the latest version.
Why is everyone buzzing... ?
The vulnerabilities that affect Joomla! will be published by the security team after solving the issue and if you don't update, hackers will easily exploit old versions .
There are movies on youtube.com about how to a hack a website with old versions of Joomla! and the forum.joomla.org is full of these topics. Usually no one is bothering to answer when they heard that you didn't updated your website because... you were too lazy.
"Please don't try to blame 'Joomla' for your lack of security if your install is out of date."