Since my medical costs are so low, I opted out of supplemental insurance and saved myself $1600 per year of premiums.
When one moves from TriCare Prime to Medicare, it becomes TriCare for Life - one pays the Medicare out of their SS, but stops paying for TriCare Prime - that makes our supplemental Insurance "free".
My Prostate Insurance was billed to TriCare at $238K, but TriCare disallowed $200K (Spent 6 hours in surgery and was discharged at noon of second day so it's obvious the hospitals overcharge the insurance knowing that they won't get the asked for amount (one of the evils of how they treat insurers as many will give heavy discounts for those who pay their own bills w/o having insurance).
When my wife sees a doctor that isn't part of the normal TriCare system, Medicare pays the 89% and TriCare picks up the rest - she's had some expensive procedures and the 20% would've added up to about $150K so far and she will likely need a knee replacement soon.
I understand that many will benefit from eschewing supplemental insurance but some of us would be paying hefty fees w/o it. Being retired Military is a big benefit for us.