Why do I have to keep my WordPress plugins up to date?

Off the top of my head I can think of four reasons:

1. Because they often introduce new functionality which sometimes makes your life a whole lot easier
2. Because they nearly always fix known bugs
3. Because WordPress core has been updated and the plugin no longer works properly with the new version of WordPress
4. And finally, and in my opinion most importantly, because the authors have plugged a security loophole which could allow hackers to find a way into your site.