Breaking traffic rules are not the only illegal things one might do that involve the use of a car where the license plate may be recorded and used to track down the offender. Spoofing plates can make it harder for police to track you down if you are committing a crime.
Also, some types of identity theft involve aggregating enough data about a person to convince someone that you are that person. A licens plate is one type of datum that may be useless on its own but could be combined with a street address to compromise someone's car insurance data, which could then lead to a VIN and title info and from there a car loan and then perhaps financial information, as just one example.
Finally, one would not want to fall victim of a failure of imagination. Even if no one right now could come up with a malicious use for a license plate number, that doesn't mean someone won't come up with one in the future. Better safe than sorry.