The best things on the Internet are free. Open Source refers to software and code that anyone can use for free. Many corporate sites (ex: Popular Science and The Onion) use Open Source to power their infrastructure. If a developer aspires to build your site using a CMS he made in his basement, shut him down because once he moves to Costa Rica to find himself, you're screwed. In most cases, you want a site that's built on a widely developed platform so that any qualified developer can manipulate it. The most cost-effective way to ensure this is to use Open Source.