I’ve worked primarily on the client side. In the past few years I’ve been the lead on five different web development / redesign projects. And I can’t count the number of agencies/firms that tried to sell me on their proprietary CMS of choice. That’s a singluar there.

While you’ve made a very reasoned argument for custom content management, I can tell you from the client side that many of us are afraid of proprietary solutions because some less-than-reputable colleagues of yours in the agency world have done a good job of building tools that require long-term relationships. The benefit of an open-source framework is that if I’m pissed off at my supplier, I can move the whole project to another supplier.

As you’ve said, no one solution is going to work for every project. A competent agency will recognize that and make sure they’re aware of a variety of options – open source and proprietary. Good on you for articulating that fact so well.