What the "Rockers" link is missing telling you, which is to be expected in a sales pitch, is that although rocker side skirts do have some aerodynamic benefit, it basically is unnoticeable for a street driver and especially at speeds less than about 180 kph and up. The side skirts help to maintain a lower pressure under the car, thereby adding a bit to reducing uplift at high speed.
Now. On the other hand, they also drastically reduce the amount of road spray that comes off your front tires and hits the sides of the car. With the addition of rockers and front splash guards, you will virtually eliminate the side panels pitting rash, commonly seen due to road debris off the fronts.
At the very least, if you don't run rockers and guards, PPF the hell out of the sides of your car. That's not the cure all but it will sure help.