-Snip-
Again, needless tedium, and again, the point of a gunner is to provide said different types of ammo. Guns can utilize several different ammo types and being able to switch those as needed is quite literally, really useful, and the thing about pyramidions is you're supposed to fly those things head on, that's how the ship really works, great engage and if you've got your turning engines buffed or you're using phoenix claw you'll be able to keep your front guns aimed at who whoever is firing at you. If the pilot isn't keeping the ship pointed at the enemy they are doing something wrong.
Secondly, if you have 3 engineers banging on the hull and nothing else, you'll eventually lose balloon, your engines, and all guns, even if a situation like the one you described there's still two ships, if they see you're hurting they will go in for the kill and wreck you. If you're facing a Hwacha fish/spire/galleon they can keep disabling you or your ally, even with the long reload time there is still that second enemy ship to deal with. Sure three engineers work, but you shouldn't count on things going as smoothly, besides, pyramidions are not the best example of how a three engineer ammo loadout works, considering that a single good engineer can feasibly keep both the engines and the hull maintained. The engineer up top is typically told to maintain the balloon behind his gun while the gunner can use a wrench to help out the main engineer in pinch.
Now I want to know exactly what kind of gunners you've encountered that have given you this view on how useful they are.