a high attack ship, with it's ability to kill multiple enemy ships in a single turn at a much cheaper cost, is more cost effective.
cost effective isn't the same things as better. high attack is almost more cost effective, including in DL i'd say, at least in the short term.
however, high defense ships are more likely to live longer, and in a sense pay for themselves over time. but the thing about them is, it's hard to make enough of them to really defend your empire.
Is the real problem here that defense resets after each round of battle, instead of after each battle?
I can't imagine your ships would be so effective if it didn't...
why is this a problem? it's intentional. if defenses reset only after battle, then what'd be the point of not simply having modules to increase HP?
You can no longer effectivly protect your ships against two weapon types.
i've heard rumors of a possible bug in the way defense rolls are calculated. this could also be the root of the OP's victories. a few people have said that 'off' defenses don't ablate. that is, if you've got 100 armor defense, but you're fired on by beams, as you all probably know you get the square root of the value as beam defense (10). but some people have said this value currently doesn't depelete each round the way properly aligned defenses do. i haven't heard much more on it, because it's not an easy thing to test. but if it's true, then mixed defenses are MUCH better (and/or broken).