I used to work for a Giant Entity in one of their accounting centers. During my first stint there badges were required but not enforced. After the first Gulf War started, doors were locked 24/7 and the badge requirements were enforced. If you forgot your badge The Gatekeeper (really nice lady that took gruff from noone - even the center manager) would stop you and make you wear a bright yellow "forgotten badge" badge. It was really hard to slip by her - she'd somehow nail you even if you used the back door.

I currently work for a small company where we keep the doors locked 24/7, but a good rap on the door will bring somebody to let you in. As a small company that deals with HIPAA related data, though, no visitors are left alone.