I believe that it goes further than that.
Assume that you are with a smaller clearing fcm. Assume that you, as well as the firm's positions combined, have a fairly large position against another house.
Now assume that there has been no stealing or commingling of customer funds with your firm. However, that other house pulled a Corzine.
End of day clearing, especially when there is no floor book squaring ... you could easily be toast.
In other words, it is not simply the integrity of your house ... it is that of others.
So ends another saga.