Indeed, as someone who does use it myself despite working in a ticket office as it is easier to do it!
It works on the Sleeper because of the 100% chance of being checked and them notice if they are being abused, but on daytime trains where people could get away without being checked I could see anyone buying them.
Ideally people would be able to use a RDG site logging in with their personal details such as priv card number and postcode, to buy tickets and collect from machines which would have their card number on. I don't know if it would be worth the effort of developing though unfortunately