Please help me understand this better if I'm incorrect.
(edited for clarity)
HFT market makers play an important role in those transactions. Specifically, they make it cheaper to buy and sell stocks by (1) tightening the bid-ask spread and (2) providing more quantity at each price, so that the average cost of executing an order is less. Not only that, when markets become tighter, they actually enable transactions to occur that would not have happened before. In other words, previously where buyers and sellers would NOT have traded because the transaction costs of crossing the bid-ask spread were too high, those two parties can now trade. Specifically, without HFT, there would be far fewer than 1 billion shares traded by utilitarian traders on a daily basis.
To address the other point of providing liquidity "when the markets need it most", let's take a step back. When you say that "market makers should step in to provide liquidity [for society's benefit]", you're implying that there's some externality to lack of liquidity in financial markets (if so, this is yet another reason that we need HFT on a daily basis). Suppose that this is the case: there is some negative externality to society when markets are illiquid, as is oft to happen when things go crazy in the world. During those times, volatility is insanely high because the risk of being in any position is also insanely high. Remember, market makers get compensated (on average) for holding risk that you don't want. If risk is higher, naturally, the compensation should be also. This is manifested in higher costs of execution: spreads widen and the available quantity at each level decreases.
If you want to force HFT market makers, which are private corporations, to step in to provide more liquidity, then you are forcing these companies to pay for that externality. In effect, they would take on huge risk for far diminished expected returns. That doesn't make any particular sense to me. However, if society as a whole has this view that some private corporations need to pay for public externalities, then that should be a matter of regulation. But if that's the case, why pick on HFT in particular? Why not force McDonalds and Whole Foods to give food to hungry people during famines? Surely, there is an externality to the food industry NOT stepping in during periods of extended hunger, "just when people need it the most."
[1] http://georgewashington2.blogspot.com/2010/10/yes-70-of-us-e...
I think that's definitely part of it. The issue is that people don't know how much high frequency traders actually make per contract. The amount is pretty small (on average). Typically, each trade takes on "bite-sized" risk and expects "bite-sized" returns. Do that thousands of times a day, and your bite-sized returns grow to something reasonable, but not ridiculous. Certainly, it's not on the order of how much other forms of proprietary trading make.
I think the other issue is that people don't understand that all traders, HFT or not, take on risk when they enter a trade. A lot of people use terms like "skimming off the top" or "making money for nothing". This indicates to me that they don't realize why market makers should even make money (on average). It's simply risk transfer, and that has a price (on average).
You're right though, identifying bona fide trends are pretty difficult. If it were easy, everyone would be doing it and arbing those trends away.
True but humans have fail-safes and kill switches too, thus this is not a valid argument against HFT in favor of human market makers. At any time, humans could step in and provide liquidity during the flash crash and during other crashes.
Humans can, and do. But algorithms can't and won't.
I'll give you the long answer when I get back from work.