Stallman is to Open Source what Ralph Nader is to Liberals or Ron Paul is to Conservatives/Libertarians. He has a well-formed, strongly held but atypical opinion aimed at solving a particular problem, often at great cost in other areas.
In other words, he's the kook that says stuff that blows our credibility with moderates leaning the other way, but does so with good intentions and a kernel of truth that would go a lot farther if posed more pleasantly.
Likewise, he provides good food for thought and frames the situation clearly for people who agree or want to incorporate more of that line of thinking.