Aren't we all trying to add liquidity to the market and get paid for it? I believe HFT's just do what we do on a micro time scale.
Example, I am a speculator and buy a share of X on Monday at 57. The seller is happy I was there to provide liquidity because the next best bid was 56, and they got an extra dollar when they wanted to sell.
On Tuesday, a buyer comes along and wants my share. The current price is 58, so I sell to him and pocket the dollar for my trouble. The original buyer is happy to buy from me, because the next best offer was 59, and they saved a dollar.
Now instead of Monday and Tuesday, imagine the timestamps as 11:15:23.0001 and 11:15:23.0002, the bid/ask spread as a penny instead of a dollar, and a million times an hour. You have yourself an HFT.
Basically all I was doing is profiting from the bid/ask spread with time shift as risk- the bid on Monday and the offer on Tuesday. This also adds liquidity for the buyer and seller if I make nothing, and also if I lose a dollar on my trade. Selling short adds liquidy too, because when I attempt to profit from a time-shifted bid/ask spread, it works whether if the time is 1 hour, 1 nanosecond, or even -1 day.