I personally (when not flying competitively) like having a gunner on my ship for a few reasons, like Geo mentioned, the various ammos in various guns can give the upper hand in different situation, you can perform different tasks with one gun, example heavy clip Gatling cab disable well and most importantly, because so many people preach that engineers are better than gunners having one gunner on my ship means I can help someone level up the class which many do not want. Making for happy crew.
Although today was funny, I met one chap who wanted to go gunner on my gat art pyra, I gave him a recommended loadout to give him more optimal ammo, and his response: "What's the point in using heavy if I'm only going to be using greased" me: "Very well of you're only going to be using greased, you may as well.go buff engineer then."
But I rarely take triple engineer outside of competitive, give people the chance to level up gunner in the hopes one day, I will be given the same opportunity.