First, although not deployed in a sophisticated manner this instance, yet we can now see how and why such a personal numeric system will likely be used.
Second, it now seems pretty clear that there will be some world-wide collapse requiring this personal numeric system. It seems likely that it could be an economic collapse, but it could also be an agricultural collapse due to devastation of war. Recall, that in the day a loaf of bread will cost a day's wages.
So, it seems that we can now see how people will willingly apply the mark to themselves and why. The question is when.