Simply put, this shifts the burden of caring for sick people further onto healthy people. When the sick person in question is someone with Parkinson's or MS, that's not a problem to me. When the sick person in question is someone with diabetes/cancer/heart disease/etc. caused in part or whole by their own personal choices, I have a problem with that.
rivrrat's example above is perfectly apt. Car insurance is not a "necessity," but neither is health insurance.