As the article points out, these firms provide liquidity which makes trading faster and cheaper. Theoretically, more liquidity should also make it easier to move capital to the most profitable enterprises.Actually, the article suggests that the firms pulled out of the market, and that their actions amplified the crash. HFT is a zero sum game, and these firms just demonstrated that they exist to extract value from the market, not provide it liquidity.
[edit]:Though there are obvious differences, I feel like HFT in general is just another form of side betting, alarmingly similar, in principle to CDOs and other derivatives, in at least a few ways. They're exotic, and theirfore unregulated, they dwarf and therefore distort "real" market activity, and they appear to provide value, but the geniuses behind them underestimate the risk because they can never account for all the possible black swan events.
When "real" markets depend on these educated guesses, and the guesses turn out to be wrong, the real markets suffer. Sure we know how to create fancy new financial products to back up these guesses, and exploit the leverage provided by developments in math and computer science, but the fact remains that we still can't even identify, let alone account for what we don't know.
The good news is that because these particular bets are shorter term, in this case the immediate damage is reversible, though their were definitely consequences that were irreversible, and if we keep allowing our equity markets get pushed around by HFT, the next time the system hiccups, the irreversible consequences may spiral out of control.
But I don't have a very deep finance background. I'd love to hear thoughts from some people who do.