License Plates - really private information? Why is it taboo to show a license plate in a picture?

I've seen license plates blurred or blacked out. Why? When there is a cool vanity plate it is ok to take a photo, but generally it seems like taking a photo of a car and not blurring out the plate seems to be taboo. Is there any legal reason? Aren't license plates out in the open in public anyway? And isn't it hard to link a plate to an individual? - Thanks in advance, I really can't get a clear answer about it.