[jplewicke]

On the other hand, there's a very good reason why HFT liquidity has evaporated in previous crashes (such as the 2010 Flash Crash) -- the exchanges broke trades in an unpredictable fashion, so that any attempt at market-making would have opened the market maker to considerable risk.

Let's say you step in during a major market crash and buy AAPL when it's trading at $200/share. Let say it then rises some more to $300/share and you sell, only to see the price recover to $500/share. What happens if the exchange breaks your original buy at $200? Then you end up being short on the way from $300 up to $500, even though you were right about the direction of trading and contributed liquidity during market distress.

They've theoretically instituted a fix for this ( http://en.wikipedia.org/wiki/2010_Flash_Crash#Trading_curb ) which will make it clear in advance what trades will be broken, so we'll have to see how HFT and market making responds in the next crash. Bid-ask spreads tend to widen significantly during market distress, which should actually increase the profitability of being a market maker during a crash.