Long and Medium term investors should be investing broadly across the entire market.

Just a decade ago you needed to buy into a mutual fund and pay sales loads and expense ratios over 1%.

Now, you can own an ETF containing a piece of each of the Fortune 500 companies for an expense ratio < 0.1% because spreads in those 500 companies are narrow and trade constantly.

Medium term and long term investors love HFT because it saves them 0.9% in yearly expenses. If you've got 100k invested in SPY instead of some Fidelity fund, HFT is saving you $900 a year.