The ability to prove your ID (if you want), on the spot and in a transparent and reasonably secure manner is useful and I would have no problems with carrying one for this purpose. The French accept it. The Germans accept it (a little too willingly IMO). The British look at the proposals from the their own government and small a rat (really, it's a particularly smelly, nasty, state-ist, control freak of a rat) and reject it.