People always ask me why would anyone pay for Agility when they can get Joomla or Drupal for free.
Here are the main points:
Open source software is not owned by anyone, so no one takes ownership over it
No tech support
No neck to ring if something goes wrong
Spotty documentation
You really have to be an expert to sift through all of the modules and add-ins to get the ones you want
They are both very confusing and difficult to use for end users
It is hard to find a developer who is experienced enough to implement it right, and if you do, they disappear after the project
It is very difficult to keep the Drupal and PHP versions up to date which makes it harder to expand your site and and drastically reduces site performance over time
You still have to host the website and the software somewhere which costs money
Also, Dan Knauss explains how Joomla is missing the boat in CMS. He argues that Joomla doesn't even classify as a CMS:
