The bazaar style greatly accelerates development and code evolution in software. The underlying problem here is a mismatch between the tester’s and the developer’s mental models of the program. In closed-source development they’re both stuck in these roles, and tend to talk past each other and find each other deeply frustrating. Source-code awareness by both parties greatly enhances both good communication and synergy between what a beta-tester reports and what the core developer(s) know. In open-source projects, developers work on what are in effect separable parallel subtasks and interact with each other very little.”]
Source: http://www.catb.org/~esr/writings/cathedral-bazaar/cathedral-bazaar/ar01s05.html