No, they are only a initial impact cushion; they do nothing for any impacts after the first.So do air bags plus seat-belt do better than just seat-belt? I was under the impression that air bags were to make up for those people that refuse to wear seat-belts.
They are a supplement to the seatbelt system; Same with the side bags. They all work together to provide safety to the occupants.
The lower cushions in the dash are another part; to help protect your legs if there is an intrusion, or if you submarine under the seatbelt.