While there are a couple of other reasons, the use of dual fuzing increases the reliability of the weapon system. While it doesn't "double" the chances of the bomb exploding, it does greatly increase it. In the best of cases, most munitions are considered very reliable if 90% of the launched, fired, dropped munitions function on impact. The inherent safeties, as well as the potential for human error in the manufacture, preparation, loading and employment of an item all contribute to the chances for failure. Couple this with the cost of the mission, the risk to the weapon system and especially the pilot, and the cost of the second fuze seems very minor.
Just my thoughts, and they apply primarily to US airdropped munitions....