The New Yorker had an interesting article on Purell in the March 4 issue. It's behind the paywall, but here are two interesting paragraphs.

Intuition is not an infalible guide to hygiene. People in a growing number of occupations wear latex gloves while they work—a good thing, seemingly. Yet gloves, if used carelessly, can promote contamination. If you handle raw chicken with your bare hands, you will probably wash them before you open the refrigerator, grab a skillet, mince a garlic clove, or meet Gwyneth Paltrow; but if you're wearing gloves you may not, because your hands will feel as clean as they did before you picked up the meat. Valerie Curtis, a behavioral scientist at the London School of Hygiene and Tropical Medicine, has argued that the disgust we feel when we manipulate uncooked chicken livers is likely a product of natural selection: protohumans who were never squeamish about where they put their hands would have lived to bring up fewer healthy off spring. Gloves muffle the signal.

And...

I asked Don Schaffner, a food microbiologist at Rutgers, whether clean hands might have a medical downside. He said, "We might have a much healthier population if we adopted the kinds of condition that we see in many Third World countries, with poor-quality food and poor-quality water and lots and lots of germs. If we did that, we would have adults who were very healthy and had very strong immune systems. Unfortunately, the price that we would pay would be extremely high infant mortality. That's the trade-off." There's a popular belief that passing exposure to pathogens strengthens the immune system in the way that exercise strengthens the body. But immunity doesn't work that way, and, in the absence of vaccinations, inoculations, or genetic serendipity, populations "acquire resistance" to lethal illnesses the way the indigenous peoples of North and South America acquired resistance to smallpox. Furthermore, alcohol-based hand rubs do not exacerbate the spread of treatment-resistant pathogens, as the overuse of antibiotics does. Alcohol kills germs in a different way, by disrupting cel membranes, a process to which organisms are almost as unlikely to become immune as humans are to become immune to bullets. Indeed, according to the National Institutes of Health, the best strategy for combatting the spread of drug-resistant bacteria is "for everyone to keep their hands clean."

So there you go. Purell away guilt-free.