Playing devil's advocate here: if you believe the purpose of the market is to help long-term investors buy and sell at rational/efficient prices (in other words, price discovery), then why is relative volumes of HFT vs. long-term investment relavent.
I'm not saying it's the best metric, or even a good metric, but there's quantitative evidence that when new news comes out and prices move rapidly, HFT reduces the time it takes for markets to settle down after big price moves. By many measures, this makes the markets more rough, since prices move more abruptly. On the other hand, with more rapid price discovery, fewer long-term investors trade at non-consensus prices during the transition period.
I'm not saying it's the best metric, or even a good metric, but there's quantitative evidence that when new news comes out and prices move rapidly, HFT reduces the time it takes for markets to settle down after big price moves. By many measures, this makes the markets more rough, since prices move more abruptly. On the other hand, with more rapid price discovery, fewer long-term investors trade at non-consensus prices during the transition period.