We have cards at my college, but we’re never asked to show them, because we don’t draw attention to ourselves. The whole point is that if someone is causing a disturbance, they are asked to show their card; and then when they can’t show it because they don’t have one (because they’re not meant to be in the college), they get thrown out.
Really, I wouldn’t mind one bit if we had to wear them on our shirts etc. I’m not a heavy anarchist, so I guess I just do as the superiors tell me.