Dropping this here, for everyone's easy access:
https://forums.frontier.co.uk/showthread.php?t=140240
Shot for shot and likely even kills per rearm against similar sized targets, frags absolutely beat cannons in this category. Cannons are better at taking down a greater number of larger prey. This is fine, this is their strength.
While the DPS vs shields of a frag may be the highest of all kinetic weapons, this is absolutely not its strongest area, considering how vastly more effective they are against hull, and their limited reserves.
Consider this: If frags had enough ammunition to do as much shield damage per rearm as multicannons, why would anything do anything else? They would be absolutely overpowered. Their ammo reserves are very intentional.
For a lot of people frags are too inconvenient to use. Everyone who has said so is absolutely right: they are not very versatile. They are pretty exclusively hull busters - you generally do not have enough to get through a fight if you use them against shields.
That does not make them bad or wrong. They are different. They do their job extremely well. I do not feel they need an ammo extension, I believe that people need to reign in their expectations.