Pretty sure this was covered in an earlier thread, but derivatives will be good for the long term health of Bitcoin. Short sellers, for example, can halt a slide when they buy back in to cover their shorts. overall this will lead to less volatility and more practical use as a currency rather than a purely speculative vehicle.
Derivatives overall get a bad wrap out there with general public. Some derivatives are bad, some are really good. The really good ones with help Bitcoin become more mainstream and be used for larger purchases. It will help stabilize the value of the coin as well.
Let's say you want to sell cars and accept Bitcoin. The price swings could made a huge impact on your bottom line. But if you hedge the price of Bitcoin, you could safely sell cars for Bitcoins without much risk for BTC dropping 5% overnight.