Those of us on Medicare realize that the card displays our SSNs just like the Social Security card.
There was the case of the guy from LifeLock who published his, promising that his company would protect him. It didn't go well.
Lifelock CEOs Indentity Stolen 13 Times (Wired)
Therefore, we can't trust the preeminent SSN-protection company from exploitation of unauthorized disclosure of our SSNs.
Yet every doctor wants to see it. More importantly, so does the Emergency Room, which can happen without notice. We may not even be conscious in order to recite it from memory.
So what can be done to prevent unauthorized disclosure of the SSN brought about by carrying the card, like if my wallet gets lost or stolen?