Speculators are transmitting information about future prices to the present, which is a useful thing to do.

For instance, after Katrina and various other oil shocks, speculators sent oil prices up in anticipation of shortages. This sent consumers the message: "please use less oil right now."

Usually, this is a good thing, though collective insanity ("house prices never go down!" or "it's a new economy, earnings don't matter!") can cause problems.