Hypothetical situation...
You're the doctor in charge of the ICU when two ambulances arrive (at the same time), each one with a patient requiring the same amount of "x blood fraction" for the same reasons (including high risk of death)... one of the patients is a well-known blood donor and the other is a JW who is willing to accept the "x blood fraction"...
However, you barely have enough "x blood fraction" for one of the patients (NOT both)... who do you give it to?