Again, not quite on topic but I believe it is useful.
One thing to remember is that the neutral and ground are bonded, so objectionable current does flow over the grounding system.
If you stand in a mud puddle and grab a ground wire, you will get shocked. You just may not be able to feel it. (Don't try this because you may feel it, and it may kill you.)
The idea behind large sections of the code is to make sure the amount of current flowing on the ground is so small as to be negligble and preferably even not measureable.
One mental exercise I found to be very useful was to imagine standing in a mud puddle holding an old electric drill with a grounded metal case and estimating what would happen if a msssive ground fault occurred at different places in the electrical system, including places like on the cable tv and a sattelite dish.
Currently in my home, the ground bus in one of the sub panels is bonded directly to the water pipes for historical reasons. Based on the above exercies, I think that may be a bad thing.
