[valkmit]

Faster transaction times result in tighter spreads, as HFT firms compete each other on the price-time priority queue. HFT firms compete against each other on time, and while yes, that's a zero sum game, the end-result to the broader market is useful.

An analogy might be something like Uber and Lyft competing with each other for clients and drivers. From the perspective of everyone else, it doesn't matter much if they ride Uber or they ride Lyft. But the adversarial games that they play against each other [Uber and Lyft] are beneficial to both riders and drivers. Perhaps a duopoly isn't the best example, so you may extrapolate this to any industry where there's a sufficient amount of participants to keep things competitive.

[caethan]

But the spread in many-to-most stocks is limited by the sub-penny rule (SEC rules, as of 2005, say you can't have a spread < $0.01) rather than by the supply of market makers in that stock. Extra competition in those markets is negative-sum.

[Kranar]

The SEC rule is that one may not quote a spread less than 0.01, but that does not mean one can not trade with a spread less than 0.01. There are several workarounds available that are well known, the simplest is in the form of mid-point pegged orders that allow spreads down to half a penny. On top of that U.S. exchanges offer a variety of different fee combinations, including negative fees which can be used to decrease the fee even further. All HFT firms take advantage of these fees, furthermore there are liquidity enhancing programs offered by the major exchanges as well as by ETFs. These can all be used to reduce the spread of a stock below the 1 cent limit.