[fr11]

Your assumption that vaccination is binary is wrong. And your last question, you don't seem to care or know about immunocompromised people.

So: - vaccination isn't perfect in preventing diseases - some people who get vaccinated aren't immune. - in some cases, vaccination may reduce disease strength but not prevent the disease. More unvaccinated kids means greater transmission threat, i.e. greater chance of an illness that, while not as bad, can still be unpleasant. - Afraid parents may have relatives who are immunocompromised, or may be immunocompromised themselves. Vaccinated people may act as short-term carriers, even if they are immune, and unvaccinated kids are more likely to act as a source, so the combination of these can cause immunocompromised people to get sick - Same as previous, but with children too young to be vaccinated (younger siblings)

Immune compromise can come from AIDS, sure, but also cancer, transplant therapy, and other causes.

Yes, some of the fear may be unreasonable - probably being stressed out and sick isn't enough immune compromise to let you get measles (though that's probably) - but enough of it isn't.