The problem isn't that shock cannons are underpowered (though as with most ammunition-based weaponry, the sustain duration has not been balanced appropriately).
The problem is that engineering has been too overpowered since its introduction.
Fix that first and I imagine the shock cannon will have a much easier time falling into a useful niche.
I fully agree. Also, the shock cannon actually is not a bad weapon for scenarios were you don't need to stay in combat for ages.
Yes, in a CZ or RES, you want MCs, due to how much ammo they pack. In a bounty hunting mission, the shock cannon can serve you quite well. Also note that shock cannons are a great choice for illegal activity on a throwaway ship.
Get a cheap ship, minimal to no engineering and do criminal stuff. Sell afterwards. Compared to non-engineered weapons, the shock cannons do quite well. The only thing i personally would change on this weapon: make them auto-fire by default. I mean, most people who use shock cannons use a rapid fire script, a macro on the mouse or the hotas configuration or something like that. (For some time when they were new, i simply have put secondary fire onall four directions of a four way switch on my throttle. So i got maximum fire rate by twiddling my thumb. )
The art of using the weapons is not how fast you can press the trigger, but to know when to fire and when to stop again, as the spread ramps up too far and you just waste ammo and damage. Thus i think that the weapon being fully automatic and ramping up the way it does, requiring trigger discipline to use it well, would merely even the playing field among those who play with basic devices and those who have sophisticated hardware.
But all in all, as V'larr already said: first smack down engineering. Many other things will then fall in place again.