Originally it was a brilliant insight. Most numbers can be fudged, but it was super expensive to have ships sitting around empty so the index was hard to game.
What made it even more useful was that it is one of the very few leading indicators of the global macro economy. Most indicators that work are lagging, which means they tell you that something has already happened.
Since hte index measures both demand for shipping and supply it can be pushed down by either. The general consensus is that the current lows is actually due to over supply by the shipping giants like maersk.
More reading:
http://www.economist.com/blogs/economist-explains/2015/03/ec...
http://www.forbes.com/sites/timworstall/2015/02/05/why-you-s...