Fully auto weapons are designed for suppressing fire. Two smartgunners with overlapping arcs would be able to put down a hell of a cover fire while their teammates moved.
People realise we have mounted guns, right? As in you sit still and fire a BFG while everything within a hundred meters shits itself. Sure, they're normally on vehicles, but that just makes a bigger target, particularky for anti materiel rounds (since these arguments always assume the enemy is perfectly equipped for every situation).