The reason for grounding the distribution system in the United States is put forth in the NEC. They mention things like stabilizing voltages and protection against line surges and lightning strikes and unintended contact with high voltage sources... the article is 250.4 (A)(1) I think but I don't have it in front of me.
I definitely have some knowledge, but I'm looking for more. I don't understand how we can have a potential voltage on a pair of wires (like our generator scenario) and not have a potential between either one of those wires and the earth. It just seems like there would be some "charge" on the wire that would want to interact with the electrons in the earth.
My brain hurts... I need to take a break from this and regroup.