Can't win Pj!
On the subject of pure water on cars....DON'T DO IT!!!!!
Regardless of what carwash companies do, they don't care what happens after, they're just looking for the easiest way to get cars in and out.
As Easyclean said, it will speed corrosion.
The main reason actually is because you're taking off all the polish, and it's protective polymers.
You'll lose the nice shine, and base colour cars will go oxidise and go flat.
As you all know, I keep my car shiny all the time (being an ex-valeter) and I wouldn't dream of getting the merest splash of it on my paintwork.
Rog.