We all agree that there is value to liquidity and have stipulated that more frequent trading decreases bid/ask spreads. But the cost of achieving that increased liquidity may be more than any benefit. I think of increased liquidity to mean that a trade happens faster and cheaper. HFT may be providing faster in some cases, but they are only doing faster trading because they know they can sell it for more to someone else very quickly. Very often that is because they are front running, which I can't think of any reason could be seen as anything other than an inefficient, rent-seeking tax. If they are buying because they believe they can sell for higher in 5 minutes, that does provide some liquidity, but makes the trades more expensive for those who just want to sell today and don't care about it happening this instant or in the next few minutes.
Comparing the market maker costs of the 1950s (non-electronic, open-outcry trading) to 2015 is a spurious strawman and ignores the inevitable efficiency gains due to advances in technology that would have come without HFT skimming off the top of transactions.
One factor you raise that could be a benefit is that HFT may be promoting competition in marketplace commissions. I don't know how large that effect is relative to what it would be without the HFT firms.
