My feeling is this, based on an admittedly scant knowledge of immunology. The point of vaccination is to prevent incidences of people having the disease. It keeps the disease from spreading, it reduces its power. But by not allowing your child to get the disease, it means that if they are ever exposed to it after the vaccine loses its effectiveness, they may be likely to get it. And because chicken pox is dangerous to adults, it feels like the safer choice on an individual level to let the children get the disease.
However, letting children get the disease does nothing to battle the spread of chicken pox. Chicken pox remains alive and well and thriving as long as we encourage our children to get chicken pox. And don’t forget – it can be fatal in some cases.
So, we have to make a choice: vaccinate, and see chicken pox become an extremely rare disease, which benefits society as a whole, or let children get chicken pox, which benefits your one child (as long as they don’t lose the roulette game and die of encephalitis).
The tough part about this choice is that in order for vaccination to do its job to eradicate the disease, pretty much everyone has to do it. So as long as large numbers of people are having parties like this, the benefits of vaccination are being sabotaged, and the choice remains difficult.
@KNOWITALL It’s a tougher choice still with the flu shot, because of the high rates of mutation. The benefits are not as clear as they are with chicken pox (which is still arguable) or with MMR (in which vaccination should obviously be done).