So why the losses that can occur are yours?
Because speculation simply displaces value from the real economy to the financial sector.
What is value? Who define what has value and what hasn't? if something has no value why do some people try to get it (or to engage in it)?
And also, what the heck does it have to do with "trading money that you don't own"? margin trading is trading money that you borrow, covered by money you already have.
Bitcoin needs to gain broader acceptance by buyers and merchants to become a credible alternative to the incumbent banking system. What bitcoin does NOT need is to develop fancy speculation tools that most people do not care about.
As I said before margin trading increases liquidity at the cost of volatility. Real trade of real goods and services increases liquidity while reducing volatility.
That's why since bitcoinica came in the bitcoin world, volatility has increased... oh wait.