Since Dec. 2005 (I was a DL beta tester), I have complained the first strike advantage was exploitable! When it was announced that it was being removed in v1.2, I said at last! However, an exploitable attacker advantage remained. In a 1 on 1 battle where both the attacker and defender wipe each other out, the attacker should survive with 1 HP but in fact is restored to full health! Even without this bug, having the attacker survive with 1 HP means that high attack ships in 1 on 1 battles will almost always win. In DA things are worse since we have brought back the old (pre-1.2) first strike advantage as one of the special race abilities; so now we can again extend the exploit from 1 on 1 battles to fleet battles!
In my opinion the only advantage that should be given to the attacker is deciding who and when to fight! If both the attacker and defender deliver a lethal blow then both should be destroyed; why can't battles end in a tie!?
Even if this change is made, I agree the high cost of defense makes the all attack ships more cost effective. Recall one weapon type is optimal against two defense types but one defense type is only optimal against one weapon type. Researching weapons is more expensive than researching defenses but I only need to research one or two weapon's trees whereas he needs to research one weapon's tree and all three defenses. For the cost/time of building a single dreadnought with moderate attack and very high defense, I can build 2 all attack dreadnoughts each with a different weapon's type. At least 1 of these all attack dreadnoughts will have the optimal weapon and easy destroy the high defense dreadnought! If you place all three defenses on your dreadnought then your effective defense will not be enough to block my attack!
Only when the attacker advantage is removed and the cost of defense is lowered will defenses be worthwhile!