Good point. The brace, in and of itself, simply isn't big enough to prevent sagging. It's for torsional strength, coupled with the under floor brace that runs side to side. The doors, properly adjusted, serve to strengthen front to back stiffness. That's why it's important to properly adjust the convertible door pieces above the striker. I don't remember their proper name but I see so many that don't touch. They need to slightly come in contact when the door is closed to form a more solid link and tie the body together.
69 RS Conv,355,M20,4.10's and I don't worry about stone chips ( yet ).