A bumper sticker conveys the same or a better message to motorists, without it being on DMV records to make IRS people curious, and without an extra fee to the state...jus' sayin'
The message being said to other motorist is that I support bitcoin. The fact that is in on a vanity plate and not a bumper sticker means that I have put much more effort
into expressing this message.
So, actually, if the message you're trying to send is that you really like bitcoins, then a vanity plate is far superior to a bumper sticker. You're very much correct though, it did cost more $ and will perhaps get me audited by the IRS, but I plan on that anyway since I run my own business and have a flurry of write offs.
disclaimer: I'm not against bumper stickers. if you look at the back left corner of my car, you'll notice a BTC bumper sticker as well.