The slight inefficiency of the market creates a small time buffer that means a lot of the meaningless fluctuations of the market can be ignored (have to be), which would seem to lend stability. HFT lets computers exploit these fluctuations instead, perhaps blowing them out of proportion. And the other thing you said is true as well -- if you have the computing power it seems much more foolproof to work the market via HFT than it would be through doing actual investment research and accumulating knowledge.EDIT: Another thought came to me: Is more efficiency a good thing anyway? I can imagine that some inefficiency between market changes and stock prices could actually be beneficial for the health of the economy. Inertia is a stabilizing factor of many systems.