Why a barrel roll?
I am sitting in my hypersonic bomber, and ground troops call in an airstrike. I fly over and suddenly notice that the bombs are attached to the top of my aircraft. Its a good thing my aircraft can do a barrel roll or I would never be able to by those allies^W journalists^W people.
I am sitting in my supersonic fighter and a boat load of Somali pirates are rowing towards towards my carrier. They suddenly appear on my radar as they leap into the sky. A launch a missile and illuminate the pirates with my radar so the missile can stay on course. Its a good thing my fighter can do a barrel roll because my radar won't point upwards and those pirates are gyrating all over the sky.
I am in my space plane on my way to moon base alpha when all of a sudden there is a massive nuclear explosion at the base. There is a big heat shield on the belly of the plane. Luckily I can do a barrel roll and duck and cover behind that.
Seriously, is there a reason for a modern aircraft to be able to do a barrel roll?