Borrowing a point from tptacek, Vanguard has come out and said that they have lower costs due to HFT. Their customers save money over the previous providers of liquidity.
Awesome, I appreciate the link. It's still not totally clear to me how/why this helps Vanguard, or whether the pros outweigh the cons for the market as a whole, but this is in fact the type of evidence that I've been asking for. Thank you for sharing!
It helps Vanguard because they don't make money through active trading: they are literally the vanguard of the movement, now practically accepted as orthodoxy, that funds should be passive and diversified across the whole market.
If you buy and hold, HFT helps you, by reducing the cost of execution --- both by reducing the spread, which is a tax you pay any time you place a market order, and by literally reducing trading fees. In fact, even if you're an active trader, HFT usually helps you: the only people truly harmed by HFT are the ones who tried to make a living selling liquidity before, who are now being outcompeted.