For any serious encounter, it's just too risky to charge a mob with a shotgun. Daily, weekly, and nightfalls with elemental damage and/or light switch promotes picking off enemies one by one from a safe distance. That being said, there are very specific situations where I do use shotguns because they're great for staggering majors/ultras. Specific instances: clearing the minotaurs out of the Nexus room; taking care of Knights in the entrance of last room in the Wakening mission; and Swordbreaker with raid gauntlets increasing reload speed is fantastic for traversing the abyss.