It still seems ridiculous to me that it's somehow sensible to run a market this way, though. Why not have an agreement among the markets that the market's active bid price will an average over the previous 5 seconds, and the ask price similarly over the previous 5 seconds. Or something similar. Why have a system that depends so much on speed when people in the real world don't?
How do fast and wild price swings help the stockowners or the companies or the economy? They only seem to help the brokers who can play the spread in quasi-real time.
It's crazy and invites wild swings by computers that will break down and will enter pathological states on occasion, as we have seen.
This reminds me... One of my old bosses found a stock that regularly cycled between $20 and $40. It did it for years on end. He said he made quite a bit of money buying at $20 and selling at $40... (Of course, the cycle eventually ended.) That was over months, though, not msec.
Cheers,
Scott.