Even more importantly certain vaccines are given at certain ages. So if some anti-science parent doesn't vaccinate their brood and say, take their kid to the same play area as my kids, one of which isn't a year old yet, she's now got an exposure to something she can't be vaccinated for yet.

This is why having high vaccination rates are so important. There is a percentage who can't be vaccinated (age, health issues, whatever), there's an unknown percentage for whom the vaccination just didn't work, and then there's the wide circle of immunity formed around them by everyone else who is immune and up to date on those immunizations.