Nobody ever said that those who use bumper stickers must only use bumper stickers. +lol+ Some people don't engage in any kind of activism at all... so, for them, bumper stickers are an improvement.

Other people do engage in different kinds of activism and, for them, bumper stickers are just a novelty.

Look, they're better than doing nothing, so what's the problem? You've explained why they don't do much good, so now explain how they can do harm.