There is one good thing about leaving it dirty - you can tell where they've been.
Example - my wife's car was getting a service one time and they were to replace air filter, check brakes, rotate tires, check fluids, change oil. It was the standard deal.
Well, it was about 25k miles or so, and I hadn't ever cleaned the engine bay. When I got home with it, I noticed the dust on the airbox was still un-fingerprinted, the wheels and lug nuts were still covered in dust with no smudges, the caps to all the fluids still had dust/grime on them, etc. It did have clean oil. So, it looked like they hadn't done anything but an oil change. My $55 receipt said they had done much more.
When I took it back to the service manager - he agreed that it looked like nothing had been done, and I got the next one free, which I never cashed in. As well as a "repeat" on that one.
Generally - I do all my own servicing. But on some cars, it is so tough to change the oil and other fluids now, its just sensible to have it done.
Oh, and I never let anyone else clean on it. No Autogeek's are in my 'hood.
DLB