While there are many ways to abuse the fact that license plates are publicly-visible (the aforementioned abortion protester being one example), there are still very legitimate reasons they should continue to be that way (holding people responsible for their actions when they infringe on the rights of others, as in my example). This isn't to say that we should always and forever attach a stamped piece of metal to cars, but the current system does have the advantage of being compatible with the Mk. I Eyeball.