However, what do you do in terms of protecting people's accounts? You say that the money is stored in your account? Gasp! How do we know you can't turn around and take all the money?
Also, what do you do to protect the accounts from a single rogue trader? If someone deposits money, margins up and loses a bunch of money, how do you protect the rest of the accounts?
For the margin trading problem, we liquidate positions by force when the user's net value falls below maintenance. All data is transparent and you can see how far you are from being taken over.