Quote from Minime:
I'll propose that there's a minimum spread that keeps the spread traders happy, and when it gets too narrow, they will leave the arena. When the volume goes down after their departure, the spreads will widen and new spread traders will enter the market, bringing everything back into equilibrium.
To keep the spread artificially wide might keep volume higher than it would be otherwise, but at a Cost to the arbitragers, directional traders and hedgers. The market should decide what the optimum balance is between all these elements, not the self-serving exchange.
I would surmise the volume is higher now than it would be if the spread were allowed to narrow, but less volume is fine if the spread shrinks and the overall transaction costs are lowered.