Now this is a question I find myself confronted with nearly everyday. Why should we pay this extra effort to think about complementary currency?
And I must admit there is no easy and general answer to this question. Just as complimentary currencies quite naturally come in many different kinds, the reasons for them are quite divers. And a general answer would be rather abstract. 
But everyday I find enough answers not only to continue thinking about it but to be fully enthusiastic and hopeful about the idea. Let´s see if I can share a few of them with you:

Many people feel a personal and global urgency. There is the financial crisis, the social disruptions, the ecological damage, the economic struggles. How will we as individuals get by and prosper without jobs? How will our communities get by and prosper without human values? How will our geopolitical cooperations get by and prosper without reliable recourses? How will humanity get by and prosper without a hospitable planet?

Complimentary currencies already help many communities, big and small, and the individuals related to these communities to cope with these questions.

Local Currencies (LETS and others) create a sense of belonging and regional business. 
Time-banking (operated as mutual credit) engages community members of all walks of life to participate for communal goals.
Business-to-Business currencies (WIR, C3, etc.) help SMEs to stay financially afloat in difficult times and maintain and create jobs where they are needed.
Loyalty currencies (Air-miles, etc.) proof how even consumers are willing to subscribe to many different currencies as long as benefits are clear and use is easy.
Ecological currencies (e.g. the Biwa in Japan) are issued by local authorities to engage the community for the achievement of their environmental commitments.
Reputation rating (like the stars you give on Blogs-posts and bay) help disperse and anonymous communities to develop there own code of conduct and establish trust.

There are already thousands of examples worldwide. How much more could be achieved if more people knew about it?

At the core of the concept lies one of its main strength: every currencies fit exclusively for the purpose it is designed for. But the idea in itself is scalable: there are currencies for working groups of only a few people, business-network of many hundred enterprises and online-currencies that reach out to millions. There is an undiscovered treasure of possibilities to be discovered.

But at the same time they are always only complimentary, the right mechanism for the right level of interactions. Its an example of subsidiarity par excellence: solve any problem right at the smallest appropriate level.
And as such it will also work if only those who are effected make an effort and agree. You don´t have to change the world first to make a difference around you.

Complementary currencies brake the vicious cycle of trying to fight problems with the same measures that created them. They help to loosen the grip of our financial system on our lives without assigning ever bigger debt to the generations after us.

And last they provide a way of valuing what we do and are without putting a price-tag onto it. Voluntary actions are not discouraged but acknowledged.

But what is more, complementary currencies are not restricted as a measure of damage-control in the world as it is now. The surrounding theory is coherent with so many different systematic models, that they even make sense if the world returned to a sustainable equilibrium over night. They provide the resilience and diversity of natural systems and at the same time foster what constitutes the core of our human capabilities: our collective intelligence and the beauty of what we can achieve together.

As I said before, this can only serve as an initial invitation to explore this field for yourself. I am sure you will find examples around you where complementary currencies are already used in practice, or where they might be soon. Please share when you find them!