Hi,
I think the airbag is there in the event of a low speed problem ( ie take off or alnding) to protect the people in the UCS as you are not facing the direction of travel but instead at an angle so it will prevent you I think from hitting the sidewall.
Personally I hope never to have the need for the airbag to inflate on me.
I do agree with the point that it is of no use at 40000ft
Regards
TBS