[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.