The problem with defense is you need about 3 times as many troops to gather the same number of defense points than their offense equivalent.
This means when killing 1 million of defense units, you actually kill 3 times as many as the number of units the attacker loses.
Though defense units are cheaper and take less time to train, It can have quite demoralizing effects on the defender already, especially if he's not a coiner and has to rebuild / earn them from Persians from the start.
So allowing for even more damage would probably despair them for good... :p
Maybe the problem is there actually : coiners don't care, non coiners can get desperate... And the balance is hard to find.
Also, the fortifications bonus has to be useful to justify the efforts and drachmas spent there.
Actually, a full coaliton defense garrisonned in a well fortified city is a powerful defense weapon. In this case, the attacker can get fully wiped out with minor losses on the defender side (or actually scattered among multiple defenders).
However, don't forget the attacker is the offender, and as such, is fully responsible for his troops doom...