For people who trade trends its a good indicator of where stocks that are sensitive to the market is heading.
like the folks who trade index futures, maybe ? If you cannot trade a 30-stock index, what makes you think you can deal with 500?
I'd like to see the Dow <13,000 by the end of today! Let's make it happen! Someone call in a bomb threat somewhere! I joke, about the 2nd part anyway.