http://www.youtube.com/watch?v=yLAZDTUUSdE
I've always believed that the long term viability of coral reefs is to make it profitable for locals to preserve them. Aquaculture like this is one way.
While over harvesting of wild stock is obviously detrimental, complete reliance on frag trading amongst hobbyists could be just as bad in the long run because then there would be less monetary incentive for the people who live by coral reefs to properly maintain them.