Let's say you're on the 10th story of a burning building and the fire department is below telling you to jump. They have one of those cartoon sheets to catch you but it's only rated to catch people falling from the 5th story. If you jump you'll hit the sheet at 40 mph but break through it and hit the ground at 10 mph. Do you shout at them to take the sheet away because you want to hit the ground at the full 40 mph because the sheet won't do enough good?

Edit: and just to address this specifically

> Except that in this case, the belt just comes off its anchor points and doesn't slow you down at all.

This is not true because the energy that went into breaking that anchor point is energy your body is no longer carrying. The belt did slow you down and even in breaking could still have saved your life.