What is the advantage of having two intake and two exhaust valves per cylinder?

Simple more airflow = more power

Easier to make does not insure that it is cheaper to operate, and does not provide the same amount of horse power. Four valves per cylinder has been around for a long time. It was not until the drive for fuel efficiency became so great that it has become the norm. In order to provide the same amount of flow with two valves the compression pressures reached will cause mechanical problems. If Ford research is right three valves is better, but the public impressed with unimportant facts.

So with fleet averages needing to go up, and the consumers lust for power, it has pretty much become a standard.

that would allow for easier air flow both into and out of the cylinders.

the valves can be smaller so less weight and lighter springs,also they fit in the chambers where they need to be for more effiency