Have they actually asked any blind people if this would be useful? I generally know my friends and family are around because I'm talking with them or know they are in the same room as me. If I need to talk to one of them in the house and they are not in the room it's not hard to yell for them and based on where I hear them responding from walk to that part of the house.
Yeah I can't really think of many uses. I only have one blind friend but she is very good at recognizing voices and will let you know if she doesn't identify your voice. It's good that this is an option though!
BBC Radio Four programme for blind and visually impaired people - "In Touch" - had a segment about conversations where you know the voice but cannot remember a name.