How do you feel about 'saviour siblings'?

This is in relation to http://news.bbc.co.uk/1/hi/health/8517897.stm in which a family are trying to have another baby, in order that the baby can donate bone marrow which could save their existing childs life.

http://www.bbc.co.uk/iplayer/episode/b00qvhqs/Having_a_Baby_to_Save_My_Child/