Equally though, someone who has a dusty/muddy car (esp in a city rather than rural area) probably values a car wash less than someone who actually has their car washed every week so their car looks relatively clean.

I keep my TT looking clean because I care a lot about the car and so have it washed every week. I'm probably the target customer for a service like this. I'd also be pissed, as I said above, if someone unsolicited washed my car.