For what it's worth, I'm 34 and had to get the MMR vaccine again last year because blood titers showed that I was no longer producing meaningful antibodies for rubella. Given that the average person has been vaccinated against over a dozen diseases, odds are good that at least one of them has worn off and needs to be re-upped by now.

So that's another reason the herd immunity is so important: If enough people opt out of just getting a simple shot, then now everyone else has to go through the much larger burden of periodic retesting and re-vaccining for them to be safe.