It's almost as if high frequency traders are actually providing a valuable service for the rest of the world! There seem to be two obvious problems here. First

It's almost as if high frequency traders are actually providing a valuable service for the rest of the world! There seem to be two obvious problems here. First, what actually happened was that at the point where that service would have been most valuable they stopped providing it. There seems to be a difference in value between idealized theoretical HFTs that provide liquidity at all times, and real HFTs that provide liquidity when there's plenty of it around anyway. Second, "they abruptly took X away and the results were bad, therefore X was providing something valuable" is an unsound argument -- e.g., coming off a drug cold turkey can be very bad even if the drug was doing you no good at all. It doesn't seem implausible that a sudden drop in liquidity during a market crash might be a very bad thing quite regardless of the optimal level of liquidity in general.