What is the benefit or motivation for having Class G airspace?
You can also fly Ifr without a clearance in G. 

Remember that the entire purpose of airspace is to keep us from hitting airliners. It is not about keeping GA planes apart. When you look at wx mins that is clear. The FAA gives us the freedom to fly around in low vis so long as we stay out of the areas you'd find airliners. 

-Robert CFII