Your example is the current futures market for oil. That market is great if all you care about is the price of oil. The phrase "prediction markets" is supposed to evoke a much broader class of markets. Ideally, if you care (enough) about some question, you create (ie, subsidize) a market specifically to answer that question. For example, weather futures (which do exist, at least as insurance) tell farmers that the high future price of their crop is due not to increased demand, but to difficulty in growing it, and thus they should not bid it down.