Seems misleading as when you go on Medicare and take an Advantage plan you have both plans.
You can chose an out of network doc for the Advantage, but pay more, or you can just omit telling them you have Advantage and just go on plain Part A, B, C, D xy. IMHO
I think you're mistaken. Advantage takes the place of Medicare, you can't just run back to Medicare for coverage if you have an Advantage plan. During the annual enrollment period you can switch back to Medicare or vice-versa, but you're stuck with your choice until you change coverage.