Those traders who didn't exit during the flash crash profited the most, what makes you think they will pull out the next time? On the flip side: those Market Makers that were forced to trade Facebook on the day of the IPO lost the most, when the system was totally broken, what makes you think they will handle the next problem better?

Anybody who's making a profit is almost by definition making the markets more efficient and less volatile. They buy when the price is low (pushing it up) and sell when it is high (pushing down).