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.
See http://www.red-sweater.com/blog/825/getting-pretty-lonely for opposing viewpoints.