You are assuming that the ground doesn't carry any current under no-fault, this is not strictly true. Every switching power supply leaks some to ground through its EMI filter. It's usually less than 1ma but it still happens. I have seen this float a disconnected ground up to 60 volts before from a single power supply, enough to feel a tingle. If you have a few switching power supplies, it all adds up.
I wouldn't want to be a plumber who disconnected a pipe fitting and then got hit with a few ma of current. Would be a nasty surprise, especially if you are holding something covered in molten solder at the time.