How about a lesson for lateral thinking? I'd still prefer to do without the coins and go for a all-virtual, all-plastic system. Efficiency and Usability.
How is it different from a $100 gift card from any store?
The anonymity advantage of cash is that it cycles through the register in a non-deterministic manner. If I buy a $20 bill (serial number 123) from an ATM, and it later winds up at PotMart, that is not compelling evidence I went to PotMart. If I bought a prepaid cash card and it was spent at PotMart, that indicates with decent probability that I purchased some weed.
If one was careful about it they could probably stay relatively anonymous though.
This, of course, doesn't make life any easier for anyone, except the jerks who already use cards for everything (aka the jerks I always get stuck behind in line while I wait for their little receipt to print and get signed).
I think that this highlights an interesting parody. To get increased efficiency you need to buy more expensive equipment. A cash register full of bills and coins doesn't cost much and after the initial investment you are pretty much done except for the expense of receipt paper.
However, credit cards require more complex card readers, systems to tie the card into central servers, etc.
In turn biometrics would require even more: pupil scanners, etc.
So the drive for efficiency and usability would appear to lead to even more complex devices, rather than the simple usability of handing a metal disk from one person to another person.
From the New Testament, Revelations, chapter 13: "Then I saw another beast, coming out of the earth ... He exercised all the authority of the first beast on his behalf ... He also forced everyone, small and great, rich and poor, free and slave, to receive a mark on his right hand or on his forehead, so that no one could buy or sell unless he had the mark, which is the name of the beast or the number of his name."
Equipment and I.T. systems to enforce biometric/body-tagging systems are costly, but then coins are costly too, and are subject to counterfeiting.
The ultimate rationale for a plasticless, cashless system though will be less in transaction costs than in control and elimination of counterfeit currency, money-laundering, payment in currency for corporate and political corruption. In such a system all money flows are visible to someone at some level, and corrupt transactions will become for-barter only. Of course, the central overseeing authority will become corrupt, which the Revelations passage hints at.
What you know
What you have
What you are
What you know can never be stolen from you, at least not without you being aware of it. What you have is hard to steal, but not impossible. What you are is trivial to steal.
Biometrics is the least secure method of verifying someone. It is trivial to clone, and even worse you can clone it at a distance without the person even realizing it was cloned.
So no, biometric tech is not a good way to paying someone.
However, what you know is subject to people forgetting, and it's hard to have a very long pass phrase.
What you have wins easily. And the real world bears this out - every security system everywhere uses it, credit cards, keys, drivers licenses.
Computers use what you know only because it's hard to do 'what you have' remotely.
The existence of Hacker News requires a whole lot of complicated technology. Having this conversation via smoke signal requires a much smaller investment. And yet...
Walmart, for instance, now uses a system that dispenses the coin change automatically so that the person at the cash register doesn't even have to do it themselves.