From a comment on this post at Harry's Place:
ID cards are absurd. If I have enough ID to claim an ID card I don't need one.
If the police stop me and I don't have my ID card on me the police can ask me to present my ID card at the nearest station, as they do with driving licences.
But what if I fail to turn up? How can they prove it was me that they asked? If they CAN prove it, what's the f****** point of the card?