"This is why you get, effectively, extremists on both sides." They always exist, but in a FPTP system must vote for 'their' party come what may. The effect is to weaken the power of those extremists unless they represent a large enough group as to endanger one of the main party's chance of winning specific constituencies. This is what is happening atm with UKIP; they are perceived as endangering the Tories, so Cameron is being forced to play to their tune; the same is true of the National Front in France. By contrast Muslim voters in the UK have largely been forced to remain voting for the major parties, which is helpful in encouraging integration.
The pathological case of PR taken to its logical extent is Switzerland where the same parties have formed the government in the same proportions since forever. The voters have almost no impact on government policy, except via referendums which often go against government policy, which is not a healthy way to run a country because it means your representatives are not being representative.