High-frequency trading is usually justified by pointing at increased market fluidity, which is a positive thing in principle. Whether that is enough justification for the problems it causes is clearly something that should be discussed more.
Ah right, market fluidity was their cover-up story. I'm absolutely not convinced, seeing how billions of dollars and euros are pumped into the market to increase fluidity, with questionable success.
That's the justification. If you go to e.g. zerohedge.com and find an article showing the millisecond by millisecond bidding by hft, you might come to the conclusion that the justification is hollow.
