The whole tailgating thing is interesting to me.
Last year, I left a job at a big state university where I'd been for 10 years.
We had two campuses, and I was on the "main" campus. Keycard access wasn't a huge deal, and really was only for after hours and a few specific areas of certain buildings. Since my building was a "teaching" building it was rare during the semester that I'd be there outside the open hours-it was something like 6:00AM to 11:00PM or something ludicrous like that. Even for that matter, my last few years there I primarily came and went in the morning/afternoon by the basement back door, which was keyed and I was one of the few folks with a key to it(actually held onto from before they put keycards on all the locks but that particular one).
I spent some time at the next building over, which was primarily a research building. Parts of the building were freely accessible, and parts weren't. The main ones I used were the loading dock and freight elevator, but I was escorted to other locked parts of the building(i.e the clean room) a few times.
Our downtown campus was a different story, since all the buildings were locked 24/7 and I didn't have keycard access to any of them. It was incredibly frustrating when I'd have a meeting down there, ask them to send someone down to let me in, and get told "Just follow someone in." It was basically a universal practice to the point where if someone saw you looking like you needed into the building, they'd usually offer(whether or not they knew you). It definitely would be a security issue, especially given what all went on down there, but it happened. Of course, to me if you have an appointment with someone and they escort you in/out of the building it's a bit of a different issue, but there again tagging along never really came up(unless you did something in the building to make it so).