Breathability is the key point of helmet to make sure your head could keep dry and comfort during riding. With more or big ventilation holes, you could get more airflow around the head, as well as get more nice and cool, but please keep in mind your head will exposed exceeded, means protection degree will decrease accordingly. Compared Road helmets with MTB helmets, Road helmets are require more or big ventilation holes. But also Road cyclist should consider more about weight issue.
Generally speaking, Bicycle helmets with lighter weight, its price will be more expensive. Therefore, unless you are going to attend to Cycle Race, it should minimize the weight, otherwise there is not necessary to spend extra cost to buy the lightest bicycle helmets.