I know I'm arriving to this discussion late, and also am too lazy to read through 10 pages, but I can hazard a guess of what's been said; since it's been brought up a lot.
When talking about the usefulness/uselessness of a gunner, you have to compare them to an engineer. (As the pilot is just shy of useless on anything but the helm)
The benefits of the gunner is that they can bring more ammo types.
The downside is that they can bring less engineer equipment/
Obviously the gunner is supposed to be more useful shooting a gun than an engineer, where the engineer is supposed to be more useful keeping the ship healthy.
However, when it comes to being on the gun, a variety of ammo types doesn't always prove to be a benefit. Certainly they can help if various situations, such as being on a gatling and using heavy clip for those longer range shots, and greased or such when things get close. Maybe then hopping onto the mortar and using burst or charged, or whatever really. The gunner has that variety, while the engineer does not.
In most situations, you don't see a huge benefit from a variety of ammo types. In the above situation, you would do just as well (relatively) by sticking with heavy clip at range, then using the default at close range. The TTK (time to kill) would be pretty close. And as each player can only shoot one gun at a time, you generally have each person assigned to a gun. So one can use heavy clip on the gat, and the other can use burst or such on the mortar. The difference between having a gunner and an engineer on the same gun diminishes even more.
So while the engineer can shoot just as well as the gunner in most situations, the gunner can never replace an engineer when it comes to keeping the ship afloat. The pipe wrench is ok, but can't compare with a spanner/mallet combo. And then they can't put out fires. And if they can put out fires, they can't rebuild or repair. And if they bring a buff hammer, they might as well sit in the corner when things start going critical.
This brings us to the main difference between the gunner and engineer. The engineer can benefit from using multiple tools at a time on various components, but the gunner can only use one type of ammo on a gun at a time. ie: engineer can rebuild with the spanner then slap it with a mallet, then start buffing it for good measure.
You can see it in a lot of competitive matches as well, where some teams don't even bother with having a gunner. In most matches, the gunner just acts a placeholder, someone who's supposed to be using the gun most of the time and not worry as much about fixing. It's a way for random players to help solidify their roles on the ship without having a lengthy discussion about who's going to do what, where competitive teams are organized enough to where they don't have to bother with it if they don't want to.
Long story short....
My suggestion to make the gunner more useful would actually be to undo some of the more recent changes. In the early days, gunners had items like heavy gauntlets and modified triggers, items you could use without reloading the gun. I'd either changing some of the current items, or adding new ones in that can be used in conjunction with the different ammo types. This would give gunners a definite edge over engineers when it comes to gunning, without diminishing the engineers ability to gun. They'd only be able to use two items max (an ammo and an aid).
Items could be like the old heavy gauntlets, or perhaps a modified scope to allow extra zoom, a small ammo box to give a little extra ammo (5% or so) on top of the ammo type addition/reduction, a small heatsink that reduces the chance of fire? Etc.
