Cannons are great. Small cannons don't have the same penalty vs med and lg ships that small multis do. They also have a relatively fast muzzle velocity, making them more accurate than larger cannons. If you have small weapon slots, filling them with cannons is a good choice.
People need to understand that sustained, effective DPS relies heavily on being able to put damage on your target quickly, as soon as possible, with a minimum of fuss. Elite's hull damage mechanic is an amoeboid hit point system, so DPS is everything. Cannons are awful in that regard. Perhaps they have marginally more sustained DPS on paper than multicannons -- although I'm not sure -- but their tendency to deal ~1 - 2% hull damage for no apparent reason, as well as their slow rate of fire and rather low velocity, makes them poor overall. There's also the fact that subsystems, notably power plants, are easy to shred with multicannons. Not so with cannons, in my experience.
Whenever I use cannons, I'm thinking to myself "how many multicannon rounds could I have unloaded into this guy by now? Why am I using weapons with substantially less
de facto sustained damage? I just chipped off 6% of this guy's hull -- but I could've probably shredded over 20% of his hull by now if I'd been using multicannons."
I'm not sure if hit chance is still a thing in WoW these days, but I recall it as being one of the most important elements for melee DPS. Missing, even if it was only every now and again, would significantly hurt your sustained DPS in a raid encounter because it equated to
downtime -- especially with a heavy, slow, two-handed weapon. Chance to hit therefore took precedence over stats that emphasized raw DPS, like strength, until a hit cap of some sort was achieved. In Elite, there isn't any equivalent to "hit chance" aside from player skill -- you can't install software or hardware upgrades to give you a better chance at landing a shot. You have to line up your shots and wait, which is time that could be spent hitting your target instead of suffering
downtime. Moreover, you *will* miss more with cannons, since their projectile velocity is slow enough that even slight changes in a target's flight pattern can throw your shot off.
So, there's the case against cannons. As far as I'm concerned, their only use is either stacking cannons on every hardpoint and brute forcing your way through a target with huge alpha damage, hopefully destroying its power plant within one or two volleys, or opting not to use them altogether. They're just... eugh.