This may be a bit of a surprise, but frag cannons actually have extremely respectable field endurance if you make sure you're using them correctly. If you multiply the single-shot damage by the total ammunition load, you get the following numbers. This is the total damage you can cause with each weapon system before needing to rearm:
Large FIXED multicannon: 3.925 * 2200 = 8635
Large FIXED frag: 4.57 * 12 * 183 = 10035.72 (the 12x multiplier is because each frag shot launches 12 pellets each doing the listed damage)
Huge FIXED multicannon: 4.625 * 2200 =10175
So a large fixed frag does fairly-nearly as much total damage as a huge fixed multicannon, and considerably more than a large fixed multicannon. Now consider that frags get a big DPS boost from high-capacity modding, while multicannons do not. Furthermore fixed frags are fairly practical to use, while fixed multicannons... not as much. Modded and used correctly, a frag cannon will allow you to substantially more total damage than amulticannon, and you'll probably be killing each target more quickly, too. They take more involved flying to make them work to their best, but why make a point of engaging in combat of you don't want to have to try at all?