Most large scale systems are AC because transformers are relatively cheap, low maintenance, and efficient. When the system is AC ground makes no difference.

With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.

I ran into a guy at a hardware store who ran just such a power supply attached our city's water (or was it natural gas?) infrastructure. I was incredulous, but the idea that it helped prevent corrosion did make sense.

It is! Look up "impressed current cathodic protection": you apply a small DC voltage to, say, pipelines to prevent corrosion.