Does Protesting and Extremism Work?

Does protest and extremism work? CalPundit has some solid thoughts. Don't think he factors in culture enough though. The question I'm still trying to answer is how much do extremist movements shift cultural norms?

I think its pretty clear that at times extremist movements pave the way for the eventual acceptance of ideas into the mainstream. Slavery is a great example. But how often does this happen? And what factors make an extremist movement succeed?