Why Open Source?
Given the right licensing policy, anyone can download and use a free library without making a major investment of time and other resources. Because the barrier to entry is so low, well-designed open source software libraries are used—and scrutinized—by more people in more ways than they would otherwise be, drawing in a diverse body of use cases.
A broad application base benefits a library's stability in two ways: first, it exposes the edge cases in a library's implementation that would otherwise go untested, leading quickly to a high degree of implementation stability. Second, the design is driven early on towards the most appropriate abstractions. As a result, the interface is more stable and the library more adaptable.
Finally, an open source library can have a brighter long-term future than a proprietary one. With many organizations vested in the library, responsibility for its health need not rest on a single person or group, and the option of making one's own modifications is always available as a last resort.