People don't remember that. They constantly think "Those are in the past!" about such diseases. They have never had to watch their children die or be crippled by something that was utterly preventable. They have never had to bury kid after kid after kid in tiny little graves because of something stupid like German Measles or diphtheria or smallpox. They forget that every single year MILLIONS of kids STILL die of those things outside the borders of "civilized" countries, and those people EMIGRATE to places like the US, Australia, England, France, Germany, Canada, etc. ALL THE TIME. They don't require those people to take full blood tests screening for all diseases. If you are lucky they are screened for things like AIDS but not much else. They could be carriers that then go to work at the convenience store your kid buys slushies at, or the gas station where you get your car filled up, or the grocery store handing you your cash, or any number of public contact jobs. The more people move around, the greater the likelihood that they will bring their homegrown versions of these killer diseases with them, and those of us who GOT immunized years ago won't be the ones to suffer, it will be those who have no such immunity because they never got the vaccines.
I don't trust things like the new Flu vaccine. I think it is rushed to market and poorly tested. However, the Salk vaccine for polio, the triple DTP cocktail for diphtheria, tetanus and pertussis,and the smallpox and rubella vaccines are all proven safe for decades. Why someone would put their kids in danger of getting some of those horrible killers when they are fully preventable is beyond me.