You can think of HFTs as market makers, liquidity providers, and automated price discovery. For their services, they end up being able to slice a penny off the top of a lot of orders.
>You also lose all the money that's driving technological advancements (like that fiber cable).
Isn't this circular? If they weren't capturing wealth created by other actors, they tautologically wouldn't have it to invest in the first place. That said, it's true that the money might not then be invested in this sort of technology.
With the bid-ask spread being wider, that adds a transaction cost to every trade, as you can't sell for the price you just bought. This means that trades are more expensive and riskier. HFT provides liquidity that reduces those risks.
HFT seems to me to be one of those words that means two different things and people select the version of HFT that fits their arguments. "Evil" HFT is a rent seeker that just can get in front of the trades you make without adding value, you're just paying an extra penny or something for transaction. "Good" HFT is a market maker with tons of info who just gets there first every time because they're FAST.
You can see how the amount of info HFT turns it "good" or "bad", and that's a tightly held trade secret for most of the trading firms, so it's a grey area.
Importantly, those don't exist.