Besides the moral and psychological implications of being "marked" for life (not entirely unlike the numbers tatooed on forearms of concentration camp prisoners) and the (obvious) privacy and security concerns, I am failing to see the actual advantages (no risk of losing your card or keys)?
So, let's say, instead of having to give your money and keys to someone holding a knife to your throat, you will be given a small sample of on the field micro surgery.
In any case having them inserted just over the (left) thumb doesn't seem to me such a good idea.