Yet it is not always practical to have someone in the exit row if a flight is lightly booked. What rule would you propose? What if it is one of those UAX flights on which there are only 4 or 5 people and they are all in F-- how would you handle that?
In that case the single exit operated by the FA is enough for a speedy evacuation.
Of course the problem arises when the plane is almost full and additional avenues of egress are needed to pull everybody out before BBQ time.