Shotguns used to take multiple shots to kill and Oracle before they buffed them in PvE. During the Atheon fight, the oracles are to high up to shotgun efficiently (if you don't one shot, you are wasting time by jumping). And you can still use Fatebringer for Oracles. Sure Vision of Confluence and Praetorian Foil work much better, but it's still doable. It's particularly useful fighting the adds that spawn during the Conflux waves and Oracle fight (assuming you don't go back to the top to snipe). Plus it kills supplicants faster.