[Suicide Junkie]

On a related note, both CBmod(se4) and GGmod(for se5) make big use of a pseudo-emissive damage system.

Small guns can't hurt the heavily shielded ships. But big guns blow through defenses.
Fighters are far from useless in practice. Sure, you have to design your fighters right, with just one or two heavy weapons for the bigger damage per hit. But that is perfectly reasonable.

If you consider today's armor, it follows a similar scheme.
9mm bullets are good, but if there is any armor, they just bounce off.
Rifles punch through light armor, but will still bounce harmlessly off a tank.
Modest tank shells can punch through thicker armor, but would still bounce off a battleship.
And so on.

The weapons all have their place, but if you use them against things with more armor than your weapon is rated for, it does pretty much nothing.
And if you use bigger guns (main tank gun) against things that are too small (infantry), then you waste major ammo on overkill. And, to a point, you can arm smaller units with bigger guns, or bigger units with smaller guns.

And therein lies the key to variety.