DR in PHB II - examples of backwards or limited use?

[Hypersmurf]

In particular, I thought I remembered an example using a demon and a devil, who wouldn't be able to hurt each other with their natural weapons since they both had DR/Good except for this rule.

I've certainly seen DR with Demons and Devils discussed before - but more along the lines of "That's why the Blood War's been going on so long - all their attacks are reduced by DR..."

-Hyp.

 

[Jhaelen]

But why would spells have a DR that fails to exactly the thing you're probably facing and need it against? The Invest [] Protection spells for instance have a DR #/evil, but it is exactly against evil that you probably need the DR most.

I've been wondering about that one, too, but I've come to the conclusion, that this is indeed the way it's supposed to be.

There have been other instances where I am sure the Designer got the DR wrong and used DR/<something> when what he really wanted it to protect against was <something>, rather than being bypassed by <something>. Sometimes this is obvious as in Heroes of Horror:
One of the tainted scholar prestige class's secrets 'Thicker than Water' grants DR/piercing and slashing because 'when you are injured, your blood withdraws from the wound'.
Not that it matters much in this case, since the prestige class is pretty much unusable due to brokenness...