[JTon]

The most compelling argument I've heard is that technical analysis is a self-fulfilling prophecy. I.e. If a large enough segment of players believe the accuracy of a certain indicator, and they all take positions reflecting this belief, and the sum of actions can move the market. Personally I've grown fond of behavioral analysis (I suppose that's a subsection of technical analysis), in the form of the candle stick technique. I think it works more often than not