None of that profit the mega-corp makes is economically useful activity (ask any economist) - they're simply leeches in the system, able to legally game it to their advantage (and, to everyone else' disadvantage, since trading is a zero-sum game).
I broadly agree with you, but it's not true that there is no useful work being done. The arbitrage that this high speed link enables has the direct effect of bringing HK prices in line with London prices and Wall Street prices, with lower latency. It is a benefit to each regional market that you can get the best price in the world at the click of a button and not just the best price on a particular exchange. It is of special benefit to smaller exchanges as HFT arbitrageurs transfer liquidity from the large exchange to the small exchange.