No flaming from me, but playing devil's advocate, let me compare unvaccinated children who don't experience normal childhood illnesses to unneutured pets who do not propagate. The reason? Most pets are neutured. The comparison? Most children are vaccinated nowadays, which lessenes the spread of such illnesses.
You are correct in that their exposure to disease is reduced. However, most childhood disease is relatively minor and only severely impacts those who have weakened immune systems to begin with.
I contracted most of those illnesses we now regularly innoculate against. I have to tell you, they were nothing but annoyances that never even resulted in the administration of antibiotics to "cure".
I was regulary innoculated for Typhoid and Rocky Mountain Spotted Fever. These are diseases that don't even cause a blip on the childhood disease radar screens of our present time.
I think the term for this phenomenon is "herd immunity" or something to that effect. I have read that as more and more parents do not vaccinate, the "herd immunity" is decreasing, setting the stage for possible recurrence on a wide scale of previously rare childhood illnesses.