There is a difference between the two methods of defence and the way it simulates the situation (and I don’t wanna hear that it’s just a game so it’s not important, because the HQ combat system , while simple, could be argued to be reflective of real combat).
When you roll extra defence dice for armour you're similating a probability that you successfully deflected the attack with your armour or weapon. The wall can not field an active defence though, all it can do is take the hit.
In short, the damage to the wall is dependant only on how it is hit, not how well it could stop itself from being hit. So it should only have body points and not defence dice.
I know I’m not going to change anyone’s opinion with this argument (especially since it’s an official rule, I just think MB are wrong on this occasion).