Even if it does narrow spreads, it doesn't matter at the margin. If the spread is 5 pennies on average without HFT, and 1 penny with it, investors are losing out. HFT is charging them a penny to remove a penny of randomness. If someone offered you a a game where you could either:

A) flip nickles: heads you keep get the nickle, tails your opponent gets it B) flip pennies: heads you keep the penny 98% of the time, 2% of the time some other third party gets to keep it (an HFT outfit), your opponent gets it

Would you take A or B?

Why do non-HFT buyers or sellers deserve the spread any more than HFT buyers and sellers? They pay the same fees you do.

I guess you're a guy from Nanex.

I'm curious why you think I should have to "prove it" if willing buyers are trading with willing sellers on an exchange they chose to trade on. Seems pretty anti-market for a finance company.

You made a false statement. Simple as that.

Is there a video that shows us what the spread looks like without HFT?