Only my take on it guys, but the way I see it, it's not an overly technical question, it doesn't need an overly technical answer.
Holes, Shrek, are usually an afterthought, they don't really do afterthought's in F1.
If you have a car like a Rally car or a Track car based on a production body, and it is made of parts of certain thickness which you cannot afford to replace with lighter ones the cheaper way to save weight is drill holes. Obviously the parts drilled would mainly have been put there for comfort or convenience, not strength, and ideally they would be out of the airstream.
Some exceptions to this maybe racing series where there is a rule giving minimum material thickness, i.e. use 22 gauge steel on the bodywork, and holes might be drilled because 22 gauge is not necessary in all areas. Also where size is not an issue i.e. in closed body silhouette racers kinda NASCAR type, they can make big I sections etc. with holes in for the same weight as small parts without holes, so they do, because they are stronger and they aren't tight for space inside the car.
In F1 though space saving packaging is critical so the parts are as thin and small as they can be and still do the job without breaking. if it was better to have a hole there it would have a hole.
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Unless of course we are talking about a diffuser, but then there is no such thing as a hole through a diffuser. The instant you make a hole in a diffuser it ceases to become a hole, then it is known as an 'aperture'.
