When you bet with money, you're making a prediction that will pay off. And its more reliable than polling because people can lie to a pollster but they usually know how they trade or spend their money. Intrade got the 2008 and 2009 elections right. Doesn't mean they have a perfect record but the markets are for the most part, predictable.
"Show me just what Mohammed brought that was new, and there you will find only things evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelogus