I often get the question of why open source code is important, aside from the usual benefits of code review, security auditing, and the general idea that more eyeballs makes for better implementations ?
Well, where it really shines is during support, when you are investigating a bug.
Let's say that you have a bug, and that for once it is not in your code. It seems to come from some library that is used by your software. When this happens, for example when I'm working on an iPhone application, it usually takes a long time to find the method responsible for the bug. If the libraries are closed-source, this time can be made longer simply by the fact that you are not sure what the dependencies between libraries are, and can spent time trying to pin-point the location of the problem. Once you have managed to track it, all you can do is file a bug report to the author, and hope it will be adressed. In this example case of an iPhone application, this might be a long time, if ever it gets fixed (as bugs are proritized by project managers).
In the case of an open source product, such as Jahia, you have the full source code, and you are free to modify it for your own means, or redistribute the modification under the same license. So this means that when tracking down a bug, I can not only more rapidly find the origin of the problem, but I can, if I know how to do it, correct it myself and not have to rely on external resources that may not be available at the time. Also, when debugging, it is really great to be able to trace through the code to understand what is going on. Maybe the bug is in your own code, but actually seeing the source code of the external library made you understand what was wrong in your own code a lot faster. You could ask an external consultant to work on it, and he could do it provided he has a good knowledge of the code, and help you fix the issue faster. Finally you can contribute the fix back to the author of the source code if the issue was found in an external piece of software.
So in the end, bugs get identified and fixed faster, you spend less time with bugs, the customer gets a answer faster, basically everyone wins.
This is for me one of the biggest commercial advantages of open source against closed source, and if it makes the customer happier, it means you will retain him.