"The bank in Margin Call was packaging MBS out of mortgages, not speculating on the price of commodities using derivatives to gain leverage."

Correct. That's my point.

It's not merely that you can't have one without the other ... it's that you very likely wouldn't want to eliminate the (margin call guys) even if you could.

The person you're replying to was talking about futures traders. Where did you get a rich guy's son from?

Speculating on futures and other derivatives is basically what those rich guy's sons are doing in their hedge funds. Most hedge funds get worse returns than the S&P 500.

So... then let them? If they're losing money compared to what they could by doing nothing, I don't see the problem.

Getting worse returns than the S&P 500 is still valuable, if those returns are uncorrelated.