I think arguing about the penetrative capabilities of daggers, rapiers, polearms, firearms, and the protective capabilities of various armours is missing the point. It's like arguing about the colour of the tiles in the bathroom when you haven't built the foundations of your house - it's irrelevant. I'll go over this one more time.
AC is a measure of how hard it is to injure a creature with a physical attack, it is NOT a measure of how hard it is to hit a creature with a physical attack. The people advocating that armour should provide DR miss this fundamental point. We know armour doesn't make you easier to hit, it makes you less likely to suffer injury. Duh. Consequently, under D&D combat mechanics, it is represented by AC. 3E represents difficulty in "hitting" an opponent with the concept of "touch AC".
DR is a 3E concept which provides interesting "flavour", but actually doesn't sit very well with the basic D&D combat mechanic of attack-roll-is-ability-to-injure, AC-is-resistance-to-injury. Remember in 1E and 2E, certain creatures could only be "hit", i.e. damaged, by "+1 or better", "silver", "cold iron" etc. weapons. DR n/magic or DR n/silver etc. provides a similar set of limitations on the effectiveness of attacks other than those specified, and so conceptually it is simply an adjunct to the AC-is-resistance-to-injury philosophy. The alternative of having different ACs against different kinds of weapons - which is the purist approach to AC - is more complex, so has been done away with by introducing what is the mechanically superfluous concept of DR.
DR n/- is inconsistent with the core concept of AC. Conceptually, DR n/- should actually be a bonus to AC.
Again, for those seeking realism in combat, play a game other than D&D. For those seeking to make sense of D&D's combat mechanics, AC-is-resistance-to-injury is a core concept, DR n/- needs to be fixed.
Cheers, Al'Kelhar