I think is a reflection of how switchers feel about Windows systems when they have to connect to those shared drives. Makes them feel good about their switch. And of course reinforces Mac users' bias against Windows. It's an emotional connection, better at least than an abstract disk image or a Microsoft logo.

It is no more smug than the envelope/stamp imagery used for email - it's not as if we email users look down upon those poor deluded souls who still cling to their snail mail. (do we?)