Most NPC weapons are useless as treasure, because otherwise the game economy would be ruined. After all, if all of the goblins you were fighting had valuable weapons that could be used, then of what use is the skill Blacksmith? In general, monsters can be thought of as using particularly crude and low-quality weapons that an Adventurer would break with constant use.
However, sometimes you can find NPC weapons that can be taken as treasure. You should take the tag and leave the physical representation (“phys rep”) with the NPC and then provide your own phys rep for the weapon if you wish to use it. If you disarm a weapon or shield from an NPC during a battle, you can pick up that weapon or shield and use it
for the duration of that battle even if there is no weapon tag attached (assuming that you have the skill in that particular weapon to use it.)
Once the battle is completed, you must return the phys rep to the NPC. Please be fair about this—don’t go taking all of the NPC weapons in order to prevent the NPCs from coming back into the battle as other monsters. If you are not using the weapon, you must give it back to any NPC who is out-of-game and needs a weapon to come back into game as another monster.