@umbra21 - That's interesting, because I've heard that they often ask the family of chemotherapy patients to give blood, so that they have some on hand for the patient if they need it for anemia.
Obviously not all of them will be an exact match, but it's likely someone will be. And of course, the blood can always go elsewhere.
I think they also do it so that the family and friends have something to do, so they don't feel so powerless. Even if they can't do anything else to help, at least they can give blood.