HFT programs help maintain liquidity in the exchanges. You stripped them away, you get more unstable prices.
Very few people understand how HFT works.
Your typical exchange or ECN has 300 order types (NYSE, NASDAQ, etc)...
And there are only 2-3 Special Orders that matter to HFT...
The ones that put you straight to the front on the line.
If you pay the exchange large fees...
You get access to these Special Orders.
Therefore, HFT is just a massive scam by Wall Street to swindle the Middle Class.
http://www.risk.net/risk-magazine/feature/2261626/exchange-order-types-prompt-fears-of-hft-conspiracy