My guess is that sadly investors value unique intellectual property that can be protected, and they might not be too crazy about open source as a result. This is because if the company goes south the patents may be the only thing the investor can salvage.
The problem with this hypothesis is that you never hear anyone in the startup community talk about their patents and how much they are worth. Not on HN, not in the media, not on Techcrunch. You never see an "Ask HN" about how to file a patent, or license a patent, or litigate a patent. And I don't recall ever hearing a VC talk up the awesomeness of patents, or encourage a startup to waste even 0.1% of its time filing for patents instead of doing something more valuable.
If you're getting angel money you're right (because you're doing proof of concept) -- but if you're at the VC stage that's incorrect. And I've gotten that from VCs...
