As far as I could tell if you successfully disarm magical weapons it goes into their inventory instead of on the ground. No trade, non magical things can be disarmed onto the ground (sarnak war braids in OT or the urd eyes found in guk). I've not tested out filling inventory to see where the magical item goes.
I would guess successful disarm would cause the mob to attack you. Not certain if unsuccessful attempts get you aggro. I always have disarm in a macro along with bash or taunt or any other spam in combat on cooldown skills. It levels fairly painlessly by having it in a macro; not quite sense heading on a strafe key fast, but it gets the job done.
On the blue server disarming things can get rid of a proccing weapon or more hilariously can be used in hammer duels to get rid of your opponent's hammer. If you're disarming in sky while corpsing keys you can cause confusion if the weapon goes into the key bag.
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