Sun makes a complete, working version of Solaris freely available, and they sell support. It's worth noting that most people who know a lot about the open source world thought that open sourcing Solaris was just a response to losing ground to Linux, and that it was too late. We haven't seen the swell of community support for Solaris that Sun was probably expecting. But you're right -- most vendors make a basic version available under an open source license, then sell support, customization, and add-ons. I think this is positive rather than negative, because it ensures that the makers of the software have a means to make money and continue to provide the product.
With an open source development model, the basic code tends to evolve to have more features and stability when users become part of the contributing community. It's up to the vendor to create an environment that encourages participation. For example, Emu Software announced that their proprietary product, NetDirector, would be released as open source at the recent LinuxWorld Conference in Boston. NetDirector is a configuration management framework for a variety of open source server-side applications, and I think this is a very good move. Users of NetDirector tend to be technical, and many customers are able to make contributions back to the project. This raises the bar on the basic version for all users.