The major benefit of having a car appraised for diminished value is that you can get the money that you otherwise would have lost once you tried to sell or trade in the car. However, that benefit tends to shrink for people who have older cars as the diminished value isn't that large. It can be a toss-up about whether or not to file a claim for diminished value for an older car because the time spent on the paperwork and appraisal might outweigh the monetary value of the claim. However, it can be a good idea to file for a diminished value car appraisal in many cases.
It's the Principle of the Thing
First, it's the principle of the thing. Your car was in an accident, its value has been lowered, and this is especially bad for you because, if this is an older car, you're closer to trading it in for another than you would be with a newer car, assuming you tend to drive cars for as long as possible. Unless the monetary value of the claim is something ridiculously low, like in the single digits, it can be worth it to file the claim to show that you know you've taken a financial hit due to someone else's negligence.
There's Older, and There's Classic
Not to mention, there are older cars, and there are classic cars whose value might be beyond what you expected. Classic cars aren't always those souped-up, restored, vintage vehicles; sometimes they're just classic models. You could have a weathered old car and still have a valuable car because of the classic value due to its year, model, and make. In those cases, you definitely need to file a diminished value claim.
Follow Through
Once you start the claim, follow through and get the appraisal done. If you start and then decide not to complete the claim, the value of your car will only drop as time goes by. If you were to try to file another claim later on, it might not go anywhere due to how old the car is at that point and how old the damage is.
Speak with your insurance agent and find an appraiser who specializes in diminished value. Your agent may be able to recommend someone; even if they can't, they'll be able to walk you through the claim process and help you deal with the other party's insurance company. At worst, the claim could be denied, but if it's approved, you're getting paid.
