A bumper sticker has never changed anyone's mind about anything. I hope to god people aren't putting these billboards on the bumpers of their cars for that purpose. Bumper stickers are useful insofar as they identify people who I have very little in common with and people who I share things in common with but probably wouldn't ever be very close to. Why wouldn't I be close to someone who has bumper stickers on their car, even if those stickers proclaim many of my own beliefs? Well, I've never been friends with someone with a lot of bumper stickers, so I'll assume that their personalities don't mesh well with mine. In that way, bumper stickers are a fantastic time saving tool.