True, the traders often get it wrong, sometimes spectactularly.  Compare their "predictions" of the price of oil in 2008 that they made two years ago vs. predictions for that same time today.  It looks like two completely different views of reality, which, in all candor, it is.

But the commodities markets do serve a valuable purpose, in that they extrapolate the current analysis to future months.  In effect they provide a "lookahead" function for the economy.  You're right that they're often reactive, needing a smack over the head with a 2x4 before they notice what's going on, but once they get the message they provide a much needed (if belated) service.

One other thing to keep in mind about the markets is that the people buying and selling there are NOT trying to guess what will happen with market prices, but what other people will guess about prices.  That's a subtle point that many people overlook; traders don't make money by correctly guessing about fundamentals, they make money by outguessing their fellow traders about future market prices, which are in turn determined by everyone's collective guesses abtuo everyone else's guesses.