I answered this question on Quora, it has been a while and given my limited understanding of working of currency and economics. Here is my take on the topic. You can find other answers here on Quora
Its just like currency, Imagine a world where you can print unlimited money i.e. where everything is free. It would not be self sustainable. Everything has a cost associated with it i.e. No free Beer in this world. To make an ecosystem which regulates itself internally, like Quora does there should be a limiting factor. This can be external like moderators, what yahoo did. Internal like Stack overflow/Quora. When every action is associated with a cost with limiting resources, people tend to (atleast rationally speaking) will use this in a most optimized way (what is most useful for them). The system becomes self regualted...I was pretty happy with the response and the acceptance so far. Please vote my answer up if you like it..
