A lot, for a start, I can get almost any english stations ID wallets on E-Bay, where as ID cards have to have your actual picture in them and you need to be a registered member to get an ID card, and before you say only registered members can get the wallets, it only takes one or two people to quit and you will have them for sale somewhere before long, a lot harder to flog off an ID badge with someone's face on it.
But having said that I would not mind having one.