I know someone who was hit head on. The airbag deployed, but she also had a bruise on her shoulder from the seatbelt. I think they work well together. I advocate both.

I think part of the confusion might be in the US at least airbags are part of the "passive protection" which is designed to protect you if you do nothing (before airbags where afordable there were all manner of pretty bad automatic shoulder belts for cars provided the mandated protection.)

https://www.google.com/?gws_rd=ssl#q=automatic+seatbelts