That's why market makers are also called scalpers, because they skim the $0.10 spread. But in return for paying the spread, what traders get is immediacy.

The problem is, how much immediacy do traders actually want? HFT provides immediacy in microseconds; its needed for continuous-time auctions because buyers sellers usually aren't in the market at the exact same microsecond interval (while the HFT is there at every interval).

Frequent Batch Auctions[1], where orders are matched in batches with single-price clearing (call auction style) - just like the opening and closing sessions of NYSE, but much more frequently with shorter periods of say 30 seconds, 1 second, or even 100ms per batch. Their models show that frequent batch auctions will reduce spreads even further (because it eliminates mechanical/latency arbitrage rents). In theory, retail traders should get even better prices (because the spreads will be smaller) if they are willing to give up microsecond immediacy for 1 second order matching.

http://faculty.chicagobooth.edu/eric.budish/research/HFT-Fre...