There are many examples in the surrounding comments. The highlights among them are.
Immunizations are not 100% effective
Babies at home too young to be vaccinated
Elderly at home that are more susceptible
People on immunosuppressants
On top of all that, there is no good reason to not immunize children. There was one study that showed a link between autism and vaccines and it was later found out that the study actually showed no link but the numbers were falsified to satisfy an agenda. Vaccines are very safe and effective at preventing disease. The rest of us shouldn't be put at extra risk because of stupid people.
Meaning, if you don't want to vaccinate your kids, that's your right, but you shouldn't be allowed to send them to school and spread the disease to other people. A few other posts near by gave examples.
Your freedom to get diseased ends at places where you are likely to spread it to others. Or, at least it should.
Just vaccinate your kids already. http://www.youtube.com/watch?v=RfdZTZQvuCo&feature=related/
He who has but four and spends five has no need for a wallet.