If your beam-only fleet can deal with enemy missiles, and some ships outrange and outrun opposing beam ships, that's better than any solution that requires you to throw expensive missiles at a problem.
Unfortunately, it's a little boring if everything works as planned.

Nobody mentioned this, but the best defense is not to be fired upon.   Engage enemy ships with your long range missiles and don't enter their AMM spam range.   If you are assaulting fortified JG, you better deal damage as quickly as possible. 

Well you will see massed laser defences in action "Wall of Light, Hull of Duranium" when I can stop being distracted by Stellaris or HoI4.  I use multiple fire controls on point defence ships generally and so far have stuck to the Dual Purpose Point Defence Array concept.  The trouble is fighting so seriously up a tech slope makes anything difficult.  You basically live on the binary distribution.

The combination of counter missiles, area point defence, final fire point defence works very well.  I would like to add armed small craft to that as they can be stationed far enough out to thin out the inbounds.

The trouble with large numbers of small missiles is that you need a lot of turrets firing on them.  So against a super salvo of 52 inbound magics the NCN is looking to have at least 104 lasers firing in point defence.  That requires 6 CLEs (12*6 = 72) plus another 34 (or in general 17 more standard combattents) arrays in support range.  Combined with 1-1 counter missile fire this probably will result in up to half the volley being stopped.  But that still means scragged ships.  The good thing is that the lasers fire every 5 seconds so each volley gets atritted.  The trouble is that small missiles move fast and that cuts your accuracy to the bone (targeting + turret speed of 12K km/s versus inbounds at 50K km/s) plus you need to see them coming in far enough out to get targeting bonuses plus to enable your counter missiles to be in space to hit the target.

As for cost if it works then the cost is "effective" if it doesn't work then the cost is "ineeffective" in my view.

The first ships swatted down 1 missile and were hit 9 times, the last ships culled 3 missiles and were hit 7 times. 
So is that with the first ships having 3 gauss cannons on a single control, and the last group with one control per cannon?
Very conclusive evidence, I stand corrected.

Just to be sure, redo that test with one ship 3 gauss turrets linked to 1 fire control each, and one ship 3 gauss turrets linked to the same fire control.
Paste what shows up in the event log, please.

I'm pretty sure that if you use final defensive fire a single fire control can take out practically infinite salvos. Of course if you also want to use it offensively you'll need to delink and delink fire controls which can get annoying, but there's no practical reason why you can't have massed beam weapons on one control for PD.

I think Sheb meant using cap 3 tech on the railguns so that they had ROF 5 instead of 15, but there does also exist spoiler tech (advanced railguns) which gives 5 shots per salvo per railgun.

Cant you have a faster firing raigun?