The major issue is HFT can create wild market swings with little to no basis in reality. It's actually possible for them to suck up all the outstanding bids over a few seconds using small amounts of capital and while a human might desire to sell if a stock goes up by 2% the seconds or minutes it takes US to make that choice is eons for the algorithms. The net result of this is actually less liquidity as someone buying or selling can't place large orders on the market or the algorithms with eat them alive. Also, they are often setup to simply stop all actions if the market deviates to far from the norm which pushes things even further out of whack.