As an example you take two cars into a dealer and both are the same age, model etc. Car one looks a total s4itter with paint swirls, chips and generally untidy bodywork. Car two had been looked after with spotless paint and it's clean. Which car will get the higher trade in price? That's the reason why some of us like our cars detailed and clean no matter how long they are kept. I haven't decided what I'm doing with mine yet come end of pcp but if I keep it I know the paintwork is nice 
Unless it's a neglected wreck, p/x is pretty much dictated by age at the local dealership (up to 3 years old). Mick once showed me a battered and chipped Viper Green Scirocco with a slightly dented wheel that he'd had for a p/x 2 days before my first Scirocco was chopped in, and they would be selling for the same price on the used forecourt (same age, same spec - 140TDI GT). I know which one I would rather buy at the used forecourt for the same price, but the dealership one won't sell the slightly crappy one any cheaper until it starts hanging around.
For big in-your-face stuff like significant dings, keyings and wheels scuffed to hell then sure, but the difference between a tidy car and a pristine car at p/x is zero (in my experience) because the sticker price difference will be zero. Could easily make all the difference in a private sale though for how quickly it sells or the difference between buying yours or the next car at the same price when yours is in better condition.
I don't sweat minor lacquer swirls over time, because they really can't be avoided long term without a lot of money being spent on frequent detailing and machine polishing, but I do refuse to allow the dealership to wash the car with less care than I would give it.
Mick didn't even look at my GTD when applying a p/x price to it, maybe he had faith that it would be as tidy as the last 2 cars he's taken in for p/x, or maybe it didn't really matter either way for what they could sell it for.
I keep my car looking as tidy as I can myself - giving it the G-Techniq treatment etc, but it would pain me to pay someone £200 every 6 months to make my car 5% shinier than I could keep it.