Ive been messing around with this just thought I'd post what I'd found out:
In all these examples both sides have 500hps: A=attack(laser) D=defence(barriers)
I chose high health to reduce the effect of first strike and somewhat high stats to reduce the effect of the 0 attack rolls
A 10A 10D vs a 20A 0D
close battle, defender seems to have slight edge (the defender being the one that has no defences)
a 20A 20D vs a 20A 0D
attacker loses half health, defender loses
a 40A vs a 20A
attacker loses half health, defender loses, so no special advantage to a matched defence.
a 10A 200D vs a 100A 0D ship
attacker loses, defender loses half health. Too much defence makes it even worse!
A 20A 10D vs a 30A 0D ship
close battle, defender seems to have slight edge
a 10A 20D vs a 30A 0D ship
attacker loses, defender loses about 2/3 health
So never have more defence than attack. And in big ship battles it is very much overrated.
However if you are up against tiny fighters then defence is the way to go!
five 10hp 10a 0d fighters vs a 50hp 50A 0D battleship
the fighters lose about half their ships, battleship is pwned. This is even more the case
when attack is much greater than hps, the fighters lose only 1 ship.
five 10hp 10a 0d fighters vs a 50hp 25A 25D battleship
battleship wipes out the fighters only losing half health.
So in summary to easily beat a high attack 0 defence capital ship use lots of fighters
to beat lots of fighters use a capital ship with balanced attack+defence
To beat a capital ship with balanced attack+defence use a high attack 0 defence capital ship, or use a different weapon.
To beat a bunch of fighters all using different weapon types, use a capital ship with all defence types. However a multi-defence capital ship is easily beaten by a high attack 0 defence capital ship.
If your tech is similar or worse than the enemies, then use fighters making sure some or all of them can penetrate the capital ships defences. This pretty much beats everything but you lose fighters every time. But trading a fighter for a capital ship is worth it. It only really works though if the battle is over in 1 round which is very common at high tech levels since hps don't increase compared to weapons.
if your tech is superior then a high attack/multi defence capital ship beats everything since there won't be any high attack capital ships to worry about.
If tech is inferior or similar then focus on multiple weapons techs, if tech is superior then research a single weapon and multiple defences.
I was getting the impression that a matched defence was supposed to always beat a matched weapon, and was confused when clearly it didn't. But once you understand that defence is designed to be used against fighters, not capital ships it all becomes much more understandable.