Political extremists are contributing to society in some positive ways. Despite how extreme some of them may seem, from time to time there are politicians that have contributed to society in positive ways. For instance, Martin Luther King contributed to society in many positive ways, even though he as an extremist at that time.
Society is a pendulum which can not always stay perfectly balanced in the middle. Those who are extreme on either side probably do not realize it but they are contributing to society by pointing out where it has gotten stuck and bringing up points for argument that may allow change to happen.
Political extremists are not contributing to society in a good and positive way. When someone takes their belief and opinion to the extreme it causes more emotion from that person and the audience. They feed off that energy creating a hateful environment towards whatever it is they decide to direct it too.
The only thing they contribute to society is harm. I guess if you want to look at them as examples not to be, well then, they do contribute. It's hard to say what a mad man can contribute to society, political figures often go too far,but the extra crazy ones, never get a good change. I know when they do, society gets Hitler's.
Political extremists are not contributing to society in a positive way. The reason being is that they come across as blowhards and too far off the beaten pass. Social progress takes baby steps, whether it's forward or backward. It's not like technology's inexorable increases. So no, extremes are bad for politics.