Unengineered they can be quite effective too, if you use a lot of them together.
A couple of people have mentioned similar things and, personally, I tend to think it's more wishful thinking than a realistic option.
First time I ever bought an Annie I tried sticking a heap of cannons on it with the idea that it'd be like Napoleonic warships in space, and I'd be delivering devastating broadsides to any ship foolish enough to come near me.
Nope.
There are a number of problems with the concept.
Firstly, they're just not powerful enough to allow you to do this.
Secondly, they don't have enough ammo to allow you to use them repeatedly on a combat ship.
Thirdly, if you fit enough cannons to be useful, you'll have no hardpoints available for other weapons.
Fourthly, related to all the above, defensively you're better off concentrating on escape instead of wasting time attempting to deal insignificant damage to an attacker.
Personally, I've come to think of cannons as something of a "sniper rifle" for ED ships.
Put a pair of 'em on something like an FdL, along with, say, a C4 laser and a pair of frag's and you've got a ship that can break shields easily, do lots of damage close-up
and snipe individual modules (such as thrusters) if the opportunity arises.
What's more, if you use cannons like this, you don't burn through the ammunition in a couple of minutes.
If I'm honest, I prefer to use rails in that role but I've fitted cannons to both my FdL and my Mamba in place of rails and they definitely do a similar job.
I tend to think rails do it better but if you want a gimballed weapon to do that job, it might be worth giving cannons a go.