[dmurray]

Does a stock market have any value to society in general?

If so, should there just be one stock market in the whole world, listing a small set of uncorrelated instruments, or is some level of competition/ variety healthy?

We haven't found any way to have those things without either HFT or something more expensive.

It's consistent and reasonable to take some kind of fringe anti-capitalist position and answer no on one of the first two questions, but if you answer yes, you end up with HFT providing value.