Belgian economist Bernard Lietaer is one of the leading researchers on complementary currencies. Here an interview with Lietaer:
Here a speech by Lietaer on money diversity:
The main point made by Lietaer that a society that relies on one single [type of] currency is unstable and by having multiple types of currency we would have a more stable economy. Different types of currency would serve different sectors of society.
Here a link to Lietear’s website: