A Defense of High Frequency Traders

By Cliff Asness et al.  Very cogent, in my view.  The bottom line:

In summary, we don't believe HFT profits are excessive or excessively consistent. We censure illegal front running as strongly as anyone, but it has near nothing to do with HFT per se. Canceling orders in the process of providing liquidity is key to any sort of market making, whether HFT or not. We support the right of HFTs, or anyone, to try to guess the direction of the market, using order flow or any other public information. We not only support the right, we celebrate the successful exercise of that right as it adds to public welfare by making markets more efficient and lowering the cost of investing. Lastly, we believe markets are "rigged" in favor of, not against, retail investors.