Yes. It creates liquidity in markets which causes markets to be far more stable and accurately priced. I would put it at worst as morally neutral.

They add liquidity to the market, so when you (or someone indirectly on your behalf) buy or sell stocks, you (they) get a better price through narrower bid-ask spreads. They also sometimes generate comedy[0].

[0]: https://www.henricodolfing.com/2019/06/project-failure-case-...

Vanguard's Chief Investment Officer said that HFTs are beneficial for them[0].

[0] https://www.sec.gov/comments/s7-02-10/s70210-122.pdf

It technically helps increase market liquidity (enabling someone to easily sell or buy new positions) and there's research that it decreases the bid-ask spread, but I don't buy it. If you believe having a small portion of society be able to easily trade stocks, then yes, but imo, I'd say it generally doesn't have any value for society in general

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.