I think background cars should be on the site. However, I agree that some limitations are needed. If there is not enough in the picture to allow a reasonable chance of identification I don't think it should be posted.

I see no point whatsoever in posting a picture of part of a fender, or a car that is so blurry or dark that no details can be seen.

Perhaps the best solution is to post everything but have some rules for deleting it after some time. If a partial or blurry picture is not identified in something like six months I would recommend that it be deleted.