GPL License = It's YOURS!
This is my favorite thing about Open Source software. Joomla is released under the GPL public license, and from a legal perspective, is YOUR software. This means that nobody can tell you what to do with it. If you don't want to upgrade it, fine. If you want to customize it to suit your needs, fine. Want to add more features? Fine. Nobody can take it back. It's not a lease or a rental - you own it!
Upgrades vs Updates:
I am NOT talking about the minor security releases that the Joomla team releases (e.g. from Joomla 1.0.14 to 1.0.15). These are important! I am talking about the platform upgrades that many people take advantage of in order to have the latest and greatest extensions (plugins) powering their site.
Joomla Platform Release Dates:
Joomla 1.0 - September 2005
Joomla 1.5 - January 2008
Joomla 1.6 - TBA
This is how it works
When the joomla team releases a new platform upgrade, the third party developers (eventually) follow suit. So IF you want the newest extensions, you should upgrade too.
So, to clarify:
1. You do NOT need to upgrade your site from Joomla 1.0 to 1.5
(I've got dozens of clients who are still running Joomla 1.0 sites just fine. There are no security vulnerabilities in this software, and if you do not need features that exist only in a newer platform, there is no reason to upgrade.)
2. You will NOT need to upgrade your site from Joomla 1.5 to 1.6
(I don't plan on using Joomla 1.6 for production sites until a few months after it is released - we can let others serve as the guinea pigs.)
3. You NEED to update your Joomla software when security patches are released.
(These updates typically only take a few minutes to install.)