However, to suggest that the entire 100 are needed to liquify when 10 will do (less than 30 seconds later I might add) is farcical.
> For large stocks in particular, algorithmic trading narrows spreads, reduces adverse selection, and reduces trade-related price discovery.
its not just a question of who will buy, its a question of who will buy at what price. if you have more liquidity suppliers, spreads will thin and you'll pay less.
I freely admit that if you want to measure liquidity as the amount of buyers in the marketplace, then HFT definitely increases liquidity. However, to suggest that HFT is the only source of liquidity, as I've said twice before, is farcical.
Also, it is totally accurate that HFTers reduce spread in the marketplace. They also increase the spread when they are driving the price of a stock up. Also, HFTers aren't unique to affecting the spread in the market. Every large scale buyer can change the spread.
Not only that, but my original point regarding the provision of liquidity and HFTers claiming they provide the essential job of supplying it (ie: without HFT, there would be no buyers) and I used the short period of time of the average stock position holding to suggest that the liquidity exists regardless of the existence of HFT.