http://www.nanex.net/aqck2/4661.html

tl;dr - high frequency traders claim that they help increase liquidity. These people found that this liquidity is largely an illusion, and that when a large order comes in, it triggers the HFT to cancel their orders before the incoming order can fill, or worse, to enter their own orders that beat out the incoming order and get filled before it can, leaving the order that *should* have been filled based on the advertised liquidity only partially filled.