I spent some time with a large customer of Alfresco's today, and heard an interesting reason for why choosing open source was critical to them. Granted, it's a large media company, and so its needs may not fit those of most other enterprise customers.
But I thought the importance it placed on open source was enlightening:
Open source is critical for us, because on our old [proprietary] content management system, we were completely dependent on the vendor if something went wrong. Alfresco's open-source CMS enables us to get into the code and start working on a fix to any problems immediately, then join up with you to ensure the fix makes its way into your supported product.
Phil Moore, formerly Morgan Stanley's executive director of UNIX Engineering, once made this point at the Open Source Business Conference, arguing that his team could provide better software support than most vendors because of its proximity to the problems. Long term, enterprises don't want to be in the support business. Short term, some of them have to be, given the critical nature of their systems.
Open source makes customers and vendors equal partners, and gives enterprises the ability to resolve immediate needs on their time, not the vendor's, when necessary. It's not for every enterprise, of course, but it just might be for you.
Disclosure: I am an employee of Alfresco.