Well, I guess I am not going to give you the theoretical answer you are looking for, but as you know we were in a few "third world" countries with a baby and selectively vaccinated. We got a bit (yes, only a bit) of grief from the Embassy doctors for not wanting to vaccinate and some support from the Embassy nurse practitioner for our choices. That 'bit' of grief was what probably led to our decision to get the few vac's we did, and if we had it to do again, we wouldn't have had any.
IMHO, I think it comes down to fear of the unknown.
In our experience, many of the people that get all bent out of shape about us traveling with a baby, don't really have any experience in the given countries.
Sorry if that wasn't what you were looking for, but I felt compelled to respond.
BTW, we are going back to India in November, and ds still isn't vaccinated and more than he was when we left.