This is a great series so far, but I think some people are taking it too seriously. This is roughly the equivalent of giving you a good tutorial about how a short map-reduce implementation works and then asking you to agree that Hadoop is awesome. It may or may not be, but the toy example isn't sufficient to make the determination. It's just background information for the uninitiated.

Some relevant questions that are completely glossed over:

1. What compels an HFT to actually trade? Is there anything forcing them to keep supplying liquidity even if the market's moving against them? How is an HFT different than an actual market maker?

2. How does an HFT decide that it has a better-than-even shot at turning a profit on a trade? Most of the objections to HFTs revolve around the answers to this question (i.e. pseudo-front-running by trying to detect large buys/sells that get split over lots of orders) and their implications (i.e. 'real' investors leaving the exchanges).

3. The "market-maker strategy" HFTs you describe are indisputably compensated for providing liquidity and taking on risk, but is the return on HFTs actually equivalent to the return on other investments with equivalent risk? If not, and they earn a premium, why isn't that evidence that something's broken?