These "middle-men" are injecting themselves between buyer and seller to take their cut simply because they have the money, resources, and -- increasingly these days -- the geographic location to beat someone else to the punch.
What is often happening is Seller A wants to sell something for $10. Buyers B, C and D submit an order to buy it for $10. The computer sees this, knows that it might be worth more than $10, and while Buyers B and C get their orders through, the computer buys it out from under Buyer D because their computers are faster. Buyer D now gets a notice that "oh, sorry, that thing you wanted is now $11."
It's the same thing as you overhearing your neighbor saying the Apple Store only has one iPhone left for sale at the Apple Store and, because you have a faster car than your neighbor, you get to the store and buy it and then offer it for resale at a higher price. That's not facilitating a transaction, and it certainly is perverting the market. When the people with the fastest computers and best geographical location get items cheapest, that's not a 'free market'.
Middle-men have classically existed to provide a _service_, adding value to a previously less valuable or more difficult transaction -- wholesalers or retailers. Wholesalers existed to pare down which goods are offered, and existed primarily because smaller retailers couldn't afford to purchase in bulk. Most 'middle men' of these classes have gone away due to the digital age and/or conglomeration. Grocery stores or websites that provide value by having multiple items in the same place that you can pay for all in one financial transaction.
What if Buyer D isn't willing to spend $11?
"only serves to make a market more efficient"
I would say it's exploiting inefficiencies. That doesn't make the market any more efficient. In fact, given the sizable number of 'computer errors' leading to significant volatility when they occur, HFT is in fact a destabilizing force in the market at times.
If Buyer D isn't willing to spend $11, then he simply doesn't buy the item. The HFT took on risk by buying the item and needs to either find someone else willing to buy it for >$10 or lose money.
>I would say it's exploiting inefficiencies.
Exploiting Market Inefficiencies == A More Efficient Market. The whole idea of "buy low, sell high" is exploiting inefficiencies in pricing, which is all HFT's are doing. As a result, they bring the buy/sell prices closer together, making it easier to trade. In return for making the market more efficient, they get a profit, and everyone wins except those who are acting in an inefficient manner. (Remember, buying something for less than it's worth is ALSO an inefficiency!)
The point about volatility is fair, but that has to do with market stability, not market efficiency. The HFTs are HIGHLY incentivized to not destabilize the market because they stand to lose a LOT of money if they make a mistake. Now, it _is_ possible someone could concoct a scheme where they profit from a destabilization caused by faulty HFT, but HFT is the means by which that actor perverted the market, not the reason the market is perverted.
There's a difference between pricing inefficiencies and technology inefficiencies. In theory there should be equal access to markets. This is why SEC laws on disclosure exist. When people with greater technological ability can, in effect, toll everyone else, you are eliminating this idea of equal access.
> The HFTs are HIGHLY incentivized to not destabilize the market because they stand to lose a LOT of money if they make a mistake.
So are all traders, but that doesn't stop things like Enron and Lehman Brothers from creating smoking holes in our economy. When you start creating legal fictions and technologies that are barely understood by those that create them, much less those that provide executive oversight, government oversight, or the general public, you are getting into very dangerous waters.
Software only works as long as the assumptions of the programmer stay valid. HFT quants are incentivized to make their companies money, not to protect the market at large. An application on a desktop computer crashing and exhibiting weird behavior isn't a big deal. A HFT process going rogue is going to cost a company millions or billions of dollars at best.