quality of life

The question often arises in medical ethics as to what is more important, the existence of life, or the quality of life. Is there anything special about life itself, so that a human being who is alive should be kept alive at all possible costs until he or she dies? This is often an opinion defended by those with religious views which make God the ultimate arbiter of who lives and who dies, so that our interference in these decisions are seen as impious interventions in the divine fiat.