OK but if the HFT traders only hold stocks for say a few seconds (or milliseconds) then that means that there is plenty of real liquidity to be had and thus HFT

OK but if the HFT traders only hold stocks for say a few seconds (or milliseconds) then that means that there is plenty of real liquidity to be had and thus HFT isn't adding value. It's adding volume, but not value. If the HFT guys are willing to hold stocks for minutes or hours or days then yeah I might concede that they're adding liquidity. From what I understand that's not HFT anymore. But for a second? I can wait an extra second for an order to get filled.