Plasma Carronades have about 3 uses
1) Cheap to research : Although other weapons will quickly rack up other advantages, carronades are simple, meaning you could put more combat power into space more quickly by prioritising them
2) JP defense : The tactical situation in a JP defense is quite well suited to PCs. The opponent will appear at relatively short range, eliminating much of the range disadvantage, and the high-alpha damage / low ROF nature of PCs is much less of a disadvantage against all but the most well trained opponents
3) Rear area garrison : There is a thread somewhere calculating the PPV / resource cost of all weapons, and PCs win. A few old hulls refitted with big, scary looking Space Cannons reassure the civs wonderfully, even if they aren't much *actual* use. Obviously, you wouldn't want to use them anywhere you think there might be a real fight, but they are the cheapest way of releasing better ships for front line duty, without the civs getting jitterry