Here's a picture of what I'm talking about.
Presumably, the idea here is to reduce the "parachute effect" caused by the rear bumper scooping up air from under the car. This still leaves you with a "partial parachute", though.
I can see something like this helping to get air out from underneath, on a race car. But road cars have an impact beam blocking airflow through those holes.
Personally, I think that encouraging air to flow through the underbody would be counter-productive, from an aero drag standpoint.