We have them too. They double as official identity photocards and stuff. Even I rate one, as a temp. Every visitor has one.
(Then again, the lock's magnetic, so you could push it open easily enough).
They work at a slight distance from the reader - so much so that I can hold my rucksack (containing said card) up to the reader and it'll let me in. This is by way of being humourous to the rest of the office.