fuindordm
Adventurer
Another good point of the AC/HP system is that it allows weak opponents to deplete the resources of strong parties, even if only a little bit. Nickels and dimes aren't impressive, but a good DM knows they can add up quickly. 
In game systems where DR and damage scale together, you end up with PCs that can only be hurt by opponents of equivalent level. After years of running a Fantasy Hero game, I started to find that limitation on encounter design very frustrating.
In D&D, damage quickly scales to over 10 hp/hit, so one or two points of DR here and there doesn't hurt the game. I actually think that 3.5 hit the sweet spot with DR: monsters don't have so much that they can't be hurt at all without the "magic key", PCs don't have so much that they can ignore opponents.
Caveat: HP may not be a per-day resource in 4e, which would change the balance of things. If PCs are expected to return to full HP very quickly after every encounter, then nickels and dimes are no longer part of the encounter currency.
I'll cash in before the metaphor becomes too strained...

In game systems where DR and damage scale together, you end up with PCs that can only be hurt by opponents of equivalent level. After years of running a Fantasy Hero game, I started to find that limitation on encounter design very frustrating.
In D&D, damage quickly scales to over 10 hp/hit, so one or two points of DR here and there doesn't hurt the game. I actually think that 3.5 hit the sweet spot with DR: monsters don't have so much that they can't be hurt at all without the "magic key", PCs don't have so much that they can ignore opponents.
Caveat: HP may not be a per-day resource in 4e, which would change the balance of things. If PCs are expected to return to full HP very quickly after every encounter, then nickels and dimes are no longer part of the encounter currency.
I'll cash in before the metaphor becomes too strained...