I dont think you realize how much damage that passive does in a fight.
(95AD) + 15.3AD from runes for a total of 110.3. So at Level 11 his passive would be dealing 240.3 TRUE Damage. So in a fight if you proc the first passive then proc it 3 times from barrels you're dealing 961 TRUE Damage. Not all the time you'll get that, but 240 TRUE damage per proc is litterally getting free ignites nevertheless the free movement speed you get for kiting
Just because "a number of Gangplank players" dont use it effective or right, doesnt mean its useless or a bad passive. Gangplank is just one of those champs that require a good amount of practice with and even then some.