"Drilling holes in the brake drum will admit road dirt and water."
True - 1 puddle and you have no rear brake at all (none, zero) for a few miles. Dirt will embed in the lining and eventually mark the drum. If left for a while, retained water will rust the drum to the shoes.
"same goes for the actual cooling effect, most restorations probably aren't ridden hard enough for that to be a problem."
True - brakes have to be up to temp to work well, too cold doesn't help. Added cooling does almost nothing except for a very long hard stop (100-0) where it might fade near the end, or repeated hard stops (road racing), or dragging the brake down a long hill. For normal use it has no effect.
For cooling, air only goes from a high pressure area to lower pressure area
. Simply making holes won't do anything - the air in the drum won't circulate. Normally, air enters through a scoop or slot in the backing plate (points forward, duh - hint: it doesn't have to be on the plate itself...), and exits through the drum holes, continually "flushing" the shoes with cooler air. The really trick designs have splitters and vanes inside the drum to make air pass both shoes before leaving.
However, it does look the part, saves unsprung weight (if swing-arm) and gyroscopic weight.