Well you can adjust the knob in a way where you can charge people for rippling through you. For example, you exchange 1 A.BTC for 0.98 B.BTCs.
I guess, but with the default set to 100% quality, probably everyone would route around me and it's not worth the risk.
This misstep gives me some concern about OpenCoin's understanding of markets. Did they seriously not realize that liquidity providing is a risky business that demands payment to the liquidity provider?