I assure you that the number of people who don't smoke because their employers test is significant. The number of people who simply don't break the law, admittedly, is probably pretty small, but they exist.
Not everyone lives in a place that offers cards.
Admittedly, I only know one person who has been arrested for possession. He was searched because campus police asked him for ID, and he didn't have any 'cause he's a Mexican illegal. Now he's been deported and took one of my best friends with him. Boo.
Still, it's something that shouldn't happen. "Not many people get caught" isn't much of an argument against legalizing marijuana.