Should point out that the NEC IS the rule.Keith B wrote:2008 National Electrical Code Section 250.52 increased the requirement to 2 metallic condcutors when using ground rods. This is to maintain the proper amount of earth contact, especially in dry or rocky soil. The rods must be properly spaced and bonded topgether with the proper AWG wire per the NEC. Here are a couple of articles:
http://ecmweb.com/bonding-amp-grounding ... -part-6-12" onclick="window.open(this.href);return false;
http://fyi.uwex.edu/mrec/files/2011/04/ ... EC2010.pdf" onclick="window.open(this.href);return false; (Electrode discussion starts on page 24)
There are two points - Provide the lowest impedance path to ground for fault currents, including lightning, and prevent electrolysis.
As far as providing the lowest impedance path to ground, it is a rule that lightning does what it wants to do, we can make suggestions and hope that lightning is nice to us and goes the way we suggest.
As far as electrolysis is concerned, EVERY piece of metal we put in or on the ground participates in it, even despite coatings (which have a tendency to fail at the most inconvenient places). I spent several years on the Northeast Corrosion Council. Our object was to coordinate between our companies (and make the effort with non-member companies) the placement of metal in or on the ground. Each month or so we would meet to discuss what was planned, what was under construction, and what sort of damage we may have found and the palliative efforts taken. One of the biggest problems with galvanic corrosion is that over time the metal itself will change chemically and the problem that started merely by contact with the earth becomes self-propagating, leading to all efforts to stop the process being in vain.
Besides doing it professionally, I also used to assist my local ham radio clubs in tracking down and fixing grounding issues. One of my favorites was the guy who bought a used aluminum tower that had been lying on the ground for a while. He disassembled it and carefully "fixed" all of the problem spots he could find, sandblasted it from end to end, and then reassembled it, carefully using stainless steel bolts at every joint.
As an aside here, one of the things that those who put metal in or on the ground and want it to survive is make sacrifices to the great god electrolysis, that is, they place "Sacrificial Anodes" at carefully calculated distances. A sacrificial anode is a metal that is more active then the metal it is meant to protect, quite often magnesium, but aluminum and zinc may be used (often in marine applications) so the the galvanic action will attack the sacrificial anode rather than the object being protected. Even railroad tracks get sacrificial anodes placed along the lines.
Stainless steel is one of the least active alloys, but in my friend's case, he was using it to fasten a very active (aluminum) tower together. AFter about a year he came to me with a couple of bolts from his tower. "Jim, this came off my tower. I noticed it was swaying a little and rattling and took these bolts out and replaced them, can you tell me what's happening?" The bolts looked like a super rat had been chewing on them, one was down to less than half its original diameter, and its color had changed to black.
Yes, I could. The very active metal of the aluminum tower, in contact with the very inactive stainless steel had led to an ionic exchange, with the aluminum literally eating away the stainless steel and also corroding away the bolt holes. We replaced all of the bolts on his tower with good hot dipped zinc ones (of a larger diameter and drilling out the holes as necessary) and he never had that problem again.
I had an inspector before power was hooked up and meter re-installed.
duplicate the effect and became obsessed 
with finding the problem because everyone was hoo-rahing them. (ie. Theyre not electricians, theyre merely arrogant light bulb changers!)