IanB said:
I think it is silly to complain about this now - chances are that the DR values scale with CR, so low CR monsters will have 5/whatever damage, so a dual wielder shouldn't have a lot of trouble getting through at those levels, then in the mid-levels it will be 10/whatever, etc. I think if a dual-wielder can't get through 15 points of DR at level 18+ then there is something seriously non-standard about the campaign or the character - and nobody says you can't make changes to things as appropriate to your game.
Oh, I'm sure an 18th level dual-wielder could get through 15 points of DR. But the real question is, can he get through the DR and still do enough damage to have an impact on the enemy?
Say this fighter has STR 30, two +5 shortswords, and weapon spec in shortsword. That gives him 1d6+17 damage with his primary attacks, and 1d6+12 with his secondary attacks.
Half the time, his secondary attacks will do no damage at all, even if they hit. His primary attack will always do damage, but it will be 3-8 points per hit. Giving him average damage, his four primary attacks will do 5.5 damage each, and his three secondary attacks will do .5 damage each, giving him a total of 23.5 damage. Against a CR 18 monster with 250+ hit points, that's a minor annoyance, at best.
Give this same fighter a +5 greatsword, he does 2d6+22 damage per hit. His four attacks will do 14 damage per hit on average, or 56 damage total. Still not great, but more than twice as good as the dual-wielder, and probably enough to make a difference in the fight. Also, don't forget the dual-wielder has a -2 penalty to hit.
The one advantage the dual-wielder has is that he can have damaging enchantments on both of his swords, which could potentially level the playing field somewhat. But the dual-wielder has to pay for that advantage in spades by having two weapons enchanted. Two +8 weapons will be well over half the suggested treasure amount for an 18th level character. Also, high-power enemies with DR are probably going to have elemental resistances too. Overall, things don't look very good for dual-wielders.