While it isn't a special perk it is still exclusive to that singular weapon in the class. It is the same with pocket infinity...you can get full auto on shotguns and scout rifles, but the only fusion rifle that has it is pocket infinity which makes it function as an exotic. Firefly on Fatebringer makes it a higher tier than other exotic hand cannons. Just because it cant destroy bosses doesn't mean it isn't overpowered against every other hand cannon (like black hammer being better than ice breaker in many situations).