[CJefferson]

No, they are not getting utility gain.

Recently I was selling an netbook, aiming to get about £100. A friend told me she was wanting something, and looking to spend about £150. We split the difference and went £125.

It would not have been a utility gain if someone had quickly jumped between us, given me £100, sold the laptop to her for £150 and kept the £50 for themselves. We would both have ended up out of pocket.

The whole point of exchanges is that they connect buyers and sellers. Parasites living making a living by being faster than anyone else are not helping anyone.

If there was a level playing field, and everyone went at the same speed, and they could still make money, then I could believe they were adding some utility.

[gaius]

It's more subtle than that. If you wanted to make the sale now and your friend wanted to buy next week when she got paid, then an intermediary would be adding value, purely by being willing to act more quickly. The difference would cover them sitting on the laptop for a week.

[a3camero]

It is yet more sophisticated for some HFT trading.

Continuing the analogy: Some trading is as if there's a person standing between the two people trying to make a deal and can hear the offers before each party can react and then make their own offers to each party to capture some of the difference (re: HFT profit).

Explanation: In some forms of HFT the key advantage is that they can buy the market data faster than other people so they can see upcoming trades before other participants (undoubtedly there are better citations but here's a decent NYT article): http://dealbook.nytimes.com/2010/06/11/opening-up-the-market...

[CJefferson]

High-speed traders are living in the millisecond range. Do you think the people doing trading on the order of seconds are pleased they are paying to save milliseconds or not? I suspect not, but would be happy to be proved wrong.

[danvet]

What would really happen is that a second trader would enter the market, offer to buy the laptop for 110$, hold onto it and then sell it on for 140$. Actual prices vary and with enough competition, will settle to something that gives the trader still a nice profit and enough cash to insure against the risk of such an operation (the trader doesn't know up-front when he can actually onload the goods and how far the price might move meanwhile).

Of course, if you're not desperate on selling right now and or the risk for the trader is too high (i.e. the minimum price you'll sell right now is higher than what the trader is willing to offer you right now), nothing happens.

Concluding, traders do add utility to a market. And as others have already said, with high freq trading it mostly results in massively reduces spreads (and the temporal utility as outlined in the laptop example essentially disappears).

[BrentRitterbeck]

These high-frequency traders are not parasites. They are providing liquidity. Essentially, they are providing you a service. If your friend had not come along, perhaps you would have been holding onto your netbook for a lot longer, or it may not have sold at all.

If you want to see what happens when liquidity dries up, you only have to look to the Flash Crash. Yes, the whole thing started due to an erroneous trade, but the fall was greatly magnified when all of the computers were pulled from the market when they couldn't make sense of things. If this type of trading were to be banished tomorrow, the market would tank so quickly that people would be begging for the computers to be flipped back on. The genie is already out of the bottle, and it can't go back in without considerable pain to everyone around it.