There is occasionally a value in multiple redundancy, also known as Plan B and Plan C. In this case, if there was some problem with parachute deployment, there was no Plan B. (Unfortunately, if the retros don't fire for Spirit and Opportunity, the same scenario holds.)
If anybody asked me, I'd try to have a system that would slow the lander down enough for the airbags to work with either Plan A (retros) or Plan B (chutes). But cost and weight are considerations that might prevent redundancy like this.