Useful coffee stains
These clever cups have foot rings in different shapes (thinking outside of the circle), so when you accidentally leave your dirty cup on the nice white linen, you're just making it more beautiful.
As someone who is trying to make it in the world as a potter, I try to make nice pots and be confident that that will sell them. Sometimes I try to figure out some sort of clever quirk that might push someone over the edge to actually buy a piece- a nice detail that shows that I paid attention - something to show that it was handmade. This stamp cup idea is definitely not an innovation that I have thought about before, and I don't actually think that I would make something with this idea.... but at least I can appreciate the cleverness.