Living in the rust belt, my main motivation for a hatch isn't to lighten the load but to get more rust-free body parts. Weight reduction is an added benefit that's nice to have.
Agreed, however weight savings in the rear is generally a bad idea considering our car's balance is like 54/46, not only will it disrupt handling but making it even harder to keep traction on an already PITA rear suspension system. That's my take on it anyway. That's why I don't care if it ghas glass, or which material is heavier, just as long as it is durable and sturdy and rust-proof.