Alan,
Years ago when I was first starting out in the trade, I asked my journeyman the same question. He told me that the reason for the 12 foot rule was that a table lamp has a cord that is 6' long. With the 12' rule, then a person could always be within the reach of a receptacle behind a large piece of furniture such as a couch/sofa without having to use an extension cord.
I do remember reading this later on in my career, but right now I can't recall exactly where it is located in printed word.
