About part Medicare B, I enrolled and maintain this because I like to visit the US for about a month a year, and in the event I end up repatriating.
That said, if one's only income is Social Security and they have no real savings, I doubt it is worth paying for as I think they would qualify for further assistance.
If one does have some savings, in the event they repatriated I think they could dump the savings in to buying a home, qualify for the assistance and hold onto their saving in equity.