As I am more jaded and cynical, I think it is in large part a desire to keep the power of the hermeneutic in the hands of the person making the KJV-only claim. He who controls the text controls the hermeneutic. And how do you argue with someone who says "that's not what it says in the original" if you don't have any way to dispute it? Say, "Well the KJV is good enough for me."
In the end, it's about control and power.