It can only lead to disaster. I once saw an episode of CSI where a family had done this-- the youngest girl was constantly in surgery, donating bone marrow and whatever else for her older brother, who had a rare disease. The oldest sister left the family because she couldn't stand the situation, and the same brother eventually killed the same sister who kept saving him because he hated that she was being used in this way. His logic was flawed, but I think the episode clearly showed the consequences of *creating* a person for the sole purpose of using him/her for another. Nobody in that family was happy, least of all the person whose life was being saved.