Why are the more experienced parents in these humorous Luvs commercials allowing their babies to "get dirty"? It's because of a theory that attempts to explain why well-developed countries like the United States have so many more people suffering from asthma and allergies compared to third world countries which suffer almost none.
Known as the "Hygiene Hypothesis," it conjectures that major improvements in public health and medical technology have led to a cleaner home environment with less infectious and parasitic disease burden resulting in less stimulation of the immune system and a consequent predisposition to allergic disease.
This hypothesis helps to explain why 3rd world nations which have poor sanitation and greater microbial/parasitic diseases do not suffer nearly as much allergic problems as well-developed nations. Basically, the dirtier and more disease ridden people grow up in, the less allergies they have because the immune system is preoccupied with fighting "real" infectious problems. In the absence of such immune stimulation, the immune system essentially gets "bored" and starts treating normal things like pollen as an infection that needs to be eliminated.
Of course, there is an alternative theory to explain this phenomenon called the hapten hypothesis. Read more about that here.