I am not sure where you are hearing or looking for statistics to assert that airbags don't help if someone is buckled in. My very first google result was from the National Institute of Health, in 1992 already had good statistics:
Driver deaths in frontal crashes were 28 percent lower in cars equipped with air bags than in cars equipped with manual lap-shoulder belts only (Citation: Zador PL, Ciccone MA. Driver fatalities in frontal impacts: comparisons between cars with air bags and manual belts. Arlington, Va.: Insurance Institute for Highway Safety, 1991)
The National Highway Traffic Safety Administration goes on to say this particular item (emphasis mine):
Concerning overall injury reduction, for serious injury, the air bag plus lap-shoulder belt (when used) and manual lap-shoulder belts alone both provided about 60 percent reduction in injury risk, while automatic belts exhibited 37 percent effectiveness when they were used. The estimated effectiveness of the air bag alone was 7 percent (not statistically significant).
This is all well and good, however, the main benefit of airbags is the brain injury prevention. This paper at PubMed specifically relates:
Air bags prevent the violent whiplash motion of the head in a frontal crash, resulting in a more controlled deceleration of the brain. Wrenching forces exerted on the cervical spine are attenuated, and the face is protected from contact with hard or lacerating surfaces. Furthermore, compliance is not a problem with air bags.
Most of the statistics I have found are prior to 1995, which is before the introduction of side and various other airbags (versus just front airbags). A critique that has been leveled at airbags in the past is that they were really only effective in front impacts (which was true prior to the introduction of the various other airbags). Monash University concludes that padding on all surfaces (such as provided by airbags) goes a great way towards reducing brain injury (although they also recommend drivers wear helmets if they really want to be safe).
Estimated harm benefits are also given for other protective measures such as air bags alone, both front and side-mounted bags, and improved seat belt systems and penetration resistant side window glazing. The benefits are presented in terms of the savings per vehicle for two discount rates, 5 and 7 per cent.