Addressing your points individually:
"The sticker really has to do with your smug sense of enlightenment, you have the answers and by slapping a bumper sticker on your car you are sharing those answers with the poor benighted rest of us."
You're probably right, although I don't feel particularly smug as much as enlightened. By the way, I applaud your vocabulary and relevant use of "benighted". It's also very humble of you to acknowledge yourself as such.
"It's nothing more than a feel-good gesture, by doing that you feel that you've contributed and don't have to roll up your sleeves and commit to any real work in that area."
What work needs to be done, and why should I have to roll up my sleeves to get everyone else to STFU about religion? If everyone would simply mind their own business in terms of religion, there'd be a lot less fighting. Conflict wouldn't be completely eradicated, but shelving public declarations of religious beliefs would go a long way.
"You probably write letters to the editor, too."
Nope. Not once. Ever.
"Has anyone worth convincing changed their mind due to a simple bumper sticker slogan? Has anyone worth convincing changed their vote based on a yard sign or bumper sticker?"
You make a very good point here. For the record, I don't like those yard signs either. Politics is like religion; people should keep their political affiliations and views private. Everyone's chance to express themselves in that manner comes in the privacy of the voting booth.