True it adds no social value, but it is arguably the ultimate hacker's game :) You can actually add a little social value by placing only orders that increase liquidity; those where someone else "takes out" your standing offer or bid. That and the fact that your profits are taxed as ordinary income in the US for equities at least.

I didn't miss that point - I just chose not to address it. Lots of things add zero (or negative) social value to the economy including Farmville or the gazillionth Instagram clone. That doesn't mean they're not worth doing.

And HFT or day-trading does bring some value to the world. It enables companies to go to the public markets and raise capital. And investors to sell their shares without having to wait too long for a buyer. But those positives come along with negatives as well.