I don't think it's very useful. Will anyone search by license plate number? If not then just leave that info in the photo. Maybe if IMCDb automatically queried all DLVA/smog/etc plate databases to get the car info, that would be nice

For one car used in various films, it'd be nice to have a feature to "link" these vehicle entries together, but the license plates are not always the same. Also, as was mentioned by others, different cars often use the same movie plate numbers.