Years ago, the national Car Care Council released a study where they followed national car auction pricing of the vehicles that were sold. One of the findings: If two vehicles identical in year, make, model, mileage and equipment went through the auction, the vehicle that was detailed, clean and cared for would bring half again as much as the dirty, unkempt vehicle. So just for economic reasons it makes sense to keep your vehicle clean.
Then, there's the value of keeping your vehicle clean for the long haul in that the, paint, body, carpets and upholstery will last longer without additional cost. If you live in a salt belt state (a state where salt and other road clearing chemicals are used to keep roads clean during the winter months) and intend to keep your vehicle for a long time, consider having a professional rust protection done to the vehicle to keep rust and corrosion down. This is best done at detail shop, not buying the quickie rust-proofing sold by the dealer when you buy the car. Finally, if rust and corrosion has set in on your vehicle, have it taken care of immediately to avert acceleration of the condition.