In the open source software community, there's considerable nervousness about paying people to work on volunteer-driven projects. For example, Joomla recently hired some developers to work on its core software, a decision that has caused much debate in the Joomla community.
There's also an understandable concern that the spirit of volunteerism will be lost or a volunteer project will be tainted when a paid staff is introduced. There are worries that a project's agenda will change to suit the needs of "privateers." However, many projects that rely completely on volunteers fall short of what can be done by a paid staff. Some projects can't afford not to make use of the benefits that a full-time, focused staff can provide.
The concept of major projects growing out of a volunteer, community-based model is not new to the world. Throughout history there are examples of pure volunteer organizations that were instrumental in the founding and formation of many projects. The building of roads is one example. While the first routes were often developed by citizen volunteers, over time the maintenance and expansion of transportation systems ultimately has been taken on by our governments and public institutions. In other words, whether we are talking about roads or technology projects, there will always be a place for volunteers, just as there is a place for professionals.
It's quite common in the software industry that great movements are started by volunteers. While this can work quite well initially, there comes a time when a volunteer-based project becomes a threat to larger, controlled organizations (for example, MySQL to Oracle, Linux to Microsoft). At that point, if the open source organization is to survive and compete, it may have to fortify its position by fostering commercial involvement that helps the project advance and compete. Red Hat is a good example. Without Red Hat, Linux might not have the strong market share it has today. It is also one of the reasons I co-founded Acquia.
Within the Drupal project, we don't have a paid staff to advance the software. However, many of the developers who contribute to critical parts of the Drupal code base make their living by building complex Drupal websites. Some Drupal developers are paid by customers to contribute their expertise to the Drupal project or are employed by companies "sponsoring" Drupal development.