This article provides an support for VoxForge's use of the GPL: The different reasons for company code contributions.   In it, the author states:

[...] licensing issues are the main reason for publishing back, but separated by very few percentage points other reasons appear: the signaling advantage (being good players), the R&D sharing, and many others. In this sense, my view is that the GPL creates an initial context (by forcing the publication of source code) that creates a secondary effect - reuse and quality improvement - that appears after some time. In fact, our research shows that companies need quite some time to grasp the advantages of reuse and participation; the GPL enforces participation for enough time that companies discovers the added benefits, and start moving their motivations to economic reasons, as compared to legal enforcing or legal risks.