The one in that pic is probably more for show than anything else.
One thing to note the photo - the side skirts would be better if they incorporated a rear tire air deflector. It looks like it tucks in right before the rear tire, directing the airflow right at the tire
For an aero-minded side skirt, it should not extend any lower than the lowest point of the vehicle. That way you are not increasing your projected area in a cross-wind, but you will keep the crosswind from directly hitting any exposed parts - kinda like an air dam for the side.
I'd say it's likely one of those "tiny but real" improvements. Partly because it's not a uniform bucket (unlike, say, a hole in the matrix of a vertical grille which you could block from either side for the same effect).