Heavy Carronade destroys balloons in one clip, Light Carronade in two clips.
Heavy Flak destroys hull in one clip, Light Flak destroys in two clips.
However, the Heavy Flak at range is very much not guaranteed as timing armor breaks with it is nigh impossible and the Light Flak can actually destroy 3/7 ships in one clip. How many times did you manage to one clip a Squid or Goldfish with a Heavy Flak? The Light Flak makes up for its lack of one clip kill ability with its ability to empty the clip very quickly and reload fast, which means that Squid, Goldfish and Spire, ships that the Light Flak can't one clip and have very low amount of armor so it breaks very easily, are in danger of being two clipped very quickly.
There are multiple guns that can't "do their job" with one clip only:
A Mercury can destroy every component with one shot only, but its armor destruction ability is very weak.
The Artemis is also good at destroying components, but is unable to quickly kill a ship once the armor is down, often due to the fact that it emptied more than half its clip before the armor breaks.
The Light Carronade, as alreay mentioned, can't destroy a balloon in one clip and although it does limit the enemies vertical mobility, there is a good chance it can't save your ally.
The Banshee in comp (or just "against good enemies") is supposed to fail miserably at its job. It can set fires? The ship probably has chem spray up and running just fine, and even though chem spray reduces repairs per second it would probably be up and running even if you don't have a banshee. But it can help stress the engineers out and disrupt chem cycles? By that time you probably already got an armor break, because Gatling and Hades. If you really wanted to kill them, the Banshee wouldn't be your gun of choice.
The Hades is not so strong. Numbers say it can easily destroy armors in one clip, which often doesn't happen, because it happens to be rather hard to hit with.
The Mine Launcher is a very unreliable and slow gun that sometimes can't do its job if the pilot has kerosene or moonshine activated so he doesn't get turned around.
And there is rarely only one ship in combat. If your ally is not a quick kill ship, you could use the Light Flak to severly damage your ally's enemy's hull, which might even kill him. The Banshee and the Artemis could potentially also do this at range, but it is much less damage output from those guns. On that Lumberjack Spire that I mentioned earlier this thread where I do have the Light Flak, said tactic of shooting your ally's enemy was working very well as we often got the kill very quickly, while disabling our own enemy with the Lumberjack, so its not a completely wrong thing to do.
There should be more reasons to put a Light Flak on your ship, much like a Mortar and Heavy Flak.
There are reasons to put a Light Flak on your ship, the problem is that the enemy often gives you enough reasons to not put it on your ship. Not being able to kill within one clip is never the primary reason, which is its lack of disable or brawling ability.