I once read the argument that Chomsky -- whether you agree or disagree with his ideas -- serves a useful purpose in delimiting a boundary of the debate. By being "radical", he allows for others to have positions that are more moderate versions of his without themselves getting pegged as radicals. Stallman served the analogous role for free software at a time when it was just not done (the GNU announcement was 1983; Wikipedia says the first nearly free BSD came out in 1991, and the Debian Manifesto was 1993). It'd be excessive to attribute Mozilla or the Free Culture movement just one person but he definitely planted the seed.
In part due to Stallman's influence, today we're not limited to arguing over the relatively minor difference of which megacorp (Microsoft? Apple? mine?) we'd like to license our computers from, but rather whether in a non-zero-sum game like software there are actually moral arguments to be had about sharing beyond simply applying capitalism.
Do I believe in or use the GPL? No, not anymore. But I do believe in free software, and still have a healthy respect for what came before.