Ran into this last night playing Skyrim... an NPC used a Dragon Shout to disarm me, something I'd forgotten can happen. I scrambled to equip another weapon and after the fight had to look around to find my fallen weapon.
While it was an interesting tactic, I am not sure it can be applied to an MMO setting.
If it were, I would think that the disarmed weapon would drop into inventory - having it drop in the world could be problematical. Even if no OTHER player could pick it up and use it, what about NPCs? I may be thinking of the Fallout games, but I seem to remember NPCs picking up weapons that their allies dropped and I wouldn't be surprised if they could pick up a disarmed weapon, too. And then there's the chance you're fighting on a slope and your disarmed weapon then sliiiiiides down the mountainside into the bay and sinks into the dark.
Another concern is accessing inventory during combat. I don't know if it's even possible in ESO; from the videos I've seen, entering inventory pans the camera around so you see the front of your character and I can't see that being helpful IN COMBAT. After a certain level (5? 15?) you can swap to an alternate-weapon build, which would help in cases where ypur primary weapon was disarmed. Could be that the secondary build was designed as, eg, a healer build instead of a damage-dealing build, though. You'd be forced to drastically change your playstyle because you were disarmed.
While I don't PvP, there's also the question of what would happen if Player A could disarm Player B. I would think, again, that the weapon would drop into (inaccessible in combat?) inventory ---- Player A would not get to keep Player B's disarmed weapon. Even so, forcing a foe to change weapons could be a valuable tactic.
IF disarms are in the game at all, that is.
PS: Just remembered another incident in Fallout-3 where I shot the pistols out of the hands of the foes facing me, crippled their gun arms and then took my time finishing them off --- since a straight-up gunfight would have been suicide. Of course, you can't disarm a spellcaster.... not sure if they have anything equivalent to crippling limbs; possibly some kind of debuff effect would take the same role...