Pro: Donors of renewable body parts—blood and bone marrow are examples—can help to save lives. The inconvenience and pain of donation are minor compared with the potential loss that would be suffered by the person who needs the transfusion or transplant but fails to get it.
Con: The violation of a person's will—even if the intention associated with that violation is the preservation of the life of another person—is too high a price to pay for the benefits that might result from harvesting a renewable part of a body.