This is a long standing D&D train of thought and I for one LOVE IT!
I mean really and farmer out there is going to be able to pick up a standard COMMON sword and slash to death the Copreal Ghost of Ravin Ous! I think not! It will take the +3 Magical Longsword of Fire to damage me.. I mean him!
But really guys, certain monsters SHOULD be above the laws of standard weapons! We see this all the time and just give it different names! I mean maybe a cool CO OP mode would also you to have Soul Damaging weapons to hurt the other player!
Yeah i know, far fetched, but you can make some really neat things this way.
Maybe there is a mob out there with super regen and there is a spell that can be enchanted on your weapon to block healing... no of this is possible without the use of resistances!
Bring it on! I will, for one, start my collection of weapons early!