Diminished Value is a legitimate claim on trucks and automobile. THe amount may be in question when dealing with the insurance company but in the eyes of the potential buyer when the vehicle is sold, it is worth less. Mention this to a dealer you might want to sell your vehicle to. "You know this car was wrecked, but it was completely repaired". The offer he made will likely go down.
You guys are insinuating there is something wrong with asking for deminished value. There is not. The amount in question is another matter. The insurance company of the at-fault party will say it is much lower. Let this same vehicle get totaled and you will get the line, 'well, you know it has been wrecked and that lowers the value'.