In theory what they are doing of value, is that at any time you can go to an exchange and say "I want to buy x" or "I want to sell y" and someone will buy it from you our sell it from you... at a price that's likely to be the accurate price.

At the extreme if nobody was providing this service, investors (e.g. pension funds), wouldn't be confident that they can buy/sell their assets as needed in size and at the right price... and because of that, in aggregate stocks would be worth less, and companies wouldn't be able to raise as much capital.

The theoretical model is: - You want to have efficient primary markets that allow companies to raise a lot of assets at the best possible prices - To enable efficient primary markets, investors want efficient secondary markets (so they don't need to buy and hold forever, but feel they can sell) - To enable efficient secondary markets, you need many folks that are in the business of XTX ... it just so happens that XTX is quite good at it, and so they do a lot of this work.

> In theory

> At the extreme

> The theoretical model

These qualifiers would seem to belie the whole argument. Surely the volume of HFT arbitrage is some large multiple of what would be necessary to provide commercial liquidity with an acceptable spread?