In my opinion, bumper stickers are just like t-shirts/patches/pins/etc. Sometimes they are to display your feelings on a topic, sometimes they are intended to be amusing, and sometimes they are to grab the attention of a certain crowd.

To me, posting a political bumper sticker only serves to show who the poster is going to/already voted for.

Personally, I do like some of the clever/amusing bumper stickers or ones that display a person's interest (as a t-shirt would do; i.e. W&B t-shirts). That said, I wouldn't put one (regardless of what it said) on my car.