You don't need Bitcoinica to short Bitcoin.
You do if you're not holding Bitcoins.
Right now, a person can only act on what they think is an overly high BTC price if they've already bought in to that extent. Similarly, a "true believer" in Bitcoin can only act on a low price if they have fiat lying around.
Futures or options contracts - or, alternatively, margin trading at a 1:1 ratio - counteract that, and let people who weren't confident to begin with act on the market when it starts to act (in their eyes) irrationally.
That's the theory, anyway. Margin trading at a higher ratio (such as the ones Bitcoinica allowed) could also allow people to just make very unwise bets and swing the market around even more irrationally than normal.