From what I remember from Greenspan's "Age of Turbulence," he mentioned that he was pleasantly surprised that the markets were able to absorb much of the shock that happened on 9/11. I believe he attributed much of that to flexibility in the markets, which is probably related to the liquidity and efficiency that are provided by, for example, high-volume traders. Greenspan argues for deregulation of such activities since they make the markets so much more flexible, and able to withstand sudden shocks such as 9/11.

Of course, much of that was before the fall '08 crisis, so I don't know how he'd respond to that :)