I was just looking at the righteous-might spell and cannot see what use the spell is if the majority of targets would easily bypass it.
then I thought... why is it that DR is always weak to the most common enemy.
what I mean is.. why do Angels have DR/evil and Devils have DR/good
... I guess it would also be a little silly if their only weakness was their allies but that would make more sense (in my mind) than your only weakness being your primary enemy.
Interesting enough, the usual creatures' alignment DR seems to place considerably higher emphasis on the good/evil axis than it does on the law/chaos attribute.
Imps (being LE) and Quasits (being CE) can fight one another for hours on end, their damage reduction not being affected by the other side's chaotic or lawful subtype.
DR/evil does not mean that evil aligned people will bypass it. It means that only evil aligned weapons will bypass it. Or enemies that have abilities with an evil subtype will surpass it.
Example: If evil cleric casts this spell it gives him DR5/good. A Paladin approaches and attacks the cleric. The clerics DR would still protect him unless the paladin used a smite, or unless he aligned his weapon.
Not quite:
Some monsters are vulnerable to good-, evil-, chaotically, or lawfully aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that matched the subtype(s) of the creature.
I guess it means that devils can go havoc all they want when going on a vacation to the prime material plane, but have to watch out once the angels arrive there, too.
... I guess it would also be a little silly if their only weakness was their allies but that would make more sense (in my mind) than your only weakness being your primary enemy.
That's because you're imagining it wrong. You're imagining it as, "I'm putting up a defense against my mortal enemy".
Instead, it's "I'm putting up a defense against everyone. Except, my mortal enemy is so horrible that he's the only person strong enough to pierce through it."
It's like a Red Dragon being weak against ice. If all he plans on fighting are ice creatures, then his immunity to fire won't do him much good at all- but that doesn't mean he should be immune to ice too. Instead, his immunity makes him great against any of the other thousands of creatures and spellcasters who use fire damage. His mortal enemy is ALWAYS going to be his mortal enemy. Alignment DR isn't supposed to be an "I win" button, it's supposed to be an "I win" button that has a specific weakness. Your enemy, of course, is going to try to exploit that weakness.
I suppose let me explain it another way. It's not the Angels saying, "The demons are attacking! Let's become resistant to the demons!"
Instead, it's the Demons saying, "Those angels are resistant to almost everything, but I know their weakness!"
It's essentially the same reason that holy swords are anathema to demons and devils. It's pure evil being used to target pure good, and vice versa.
It works the other way around to, Demons are just as vulnerable to angelic attacks.
Pretty much this. Good is to evil what negative energy is to positive energy. The opposing-force of choice.
Whatever can truly hurt you the worst stands a good chance of becoming your worst enemy.