Bruce Schneier writes on worst case thinking. He's right, don't'cha know.
What's the worst that could happen if we implemented communism? We could, I suppose, all end up living in a totalitarian dictatorship, die in a zombie apocalypse or be subjected to Justin Bieber. That's also the worst that could happen if we keep capitalism, if we implement anarchism, Libertarianism, or Utopian socialism, or switch to the metric system.
One the one hand, we don't want to do things that have a substantial, foreseeable probability of severe catastrophe. Drilling for oil 18,000 feet into the Gulf of Mexico springs immediately to mind. On the other hand, we can't cower in our beds in fear of being struck by a meteor.
There are steps we can take to mitigate and virtually eliminate the possibility of the worst effects of most endeavors; there really are only a limited number of categorically Bad Ideas that no one should seriously consider.