When the good guys fight nearly anyone, it's not so painful. Only evil outsiders, and specific champions dedicated to evil, can natively penetrate the DR.Nail said:Rrrright.....
And when all the good guys fight each other, it's not so painful.
An adventurer with a Mantle of Faith can go to war against the most evil, despicable, heinous, opressive tyrant and get benefit of his DR. Ditto if he fights marauding githyanki raiders, or yuan-ti slavers, or werewolves or yuan-ti or just about anything else in the universe. Only when nearly all his enemies are fiends does the DR become meaningless, and that's a campaign issue rather than a problem with the item itself. (A Vorpal Sword is little use if you fight nothing but undead, yet that doesn't mean it's a weak item in general.)
Consider the view from the opposing side. Your good adventuring party is battling a monster that has alignment DR with a high value, and you're having trouble doing damage. Should your paladin cry in frustration, and wish he could invoke unholy powers to hurt the monster? Or should he cast holy sword and smite the beast with divine might? I know which sounds like more fun to me.