The good thing about armor class is that even if it's a dragon, you can still wound it even at low levels. So, a ton of grunts can still kill a dragon.
Meanwhile, DR can make some enemies unkillable.
Imagine a dragon with DR. Yeah. That would be no fun.
Why is it good to be able to wound high level creatures at low levels? What's wrong with e.g., an Ancient Dragon taking no damage against 500 commoners (or 1st level fighters) with bows?
