Two weeks ago I explained how proprietary software might benefit open source projects, and began to look for examples to support a proprietary feedback hypothesis. As Paul Ramsey wrote, it's hard to get proprietary users to come out of the closet, so it's very likely that I don't have all the evidence. What I have found does not support the hypothesis of a general feedback between proprietary and open source GIS software. Any feedback is much more narrow and selective. I now have 2 new hypotheses:
Proprietary benefit for open source GIS software is primarily a phenomenon of the GDAL project. No other project has such a list of proprietary contributions, and I think this says more about Frank Warmerdam, his ingenuity, and his customer service than it does about open source GIS in general.
Proprietary benefit for open source GIS software goes almost exclusively to low-level projects. What are proprietary software companies contributing to GDAL? Drivers. Format readers and writers. Bedrock functionality. Not all contributions go to GDAL (a fact that undermines my first hypothesis a bit), but the others do go to similarly low-level projects. See the comment about GEOS in my previous post.
My conclusion is that developers of higher-level open source GIS projects shouldn't be overly concerned about scaring off contributions by choosing the GPL. I've found vanishingly little evidence that proprietary companies contribute at other than the lowest level.