Plastics are made from organic solids, and recent studies indicate that pound for pound they are a better shield against galactic cosmic rays then some metals.
If you are lifting mass to orbit or moving it around, less mass is best. Asteroids and comets are obvious sources for metals. These may prove more economical sources for metals then sources launched from Earth.
What about source materials for making plastics, are there any sources in our solar system other then Earth for raw materials?