As eluded to, I like to think of Disarm as the cheap version of Dissolve, have a large Zombie swarm plan of attack this round but don't have enough mana to Dissolve the 4 armor Wizard with Veterans belt? You could use both of your casts (well timed of course) to place one Dissolve the Chest Piece Armor and place a Disable token the Belt.
Is the FM ready to use a FA attack and he has initiative? Disarm him during the first QC and move on out before he can strike. This will work well in this scenario especially if he also (before you) cast a spell during the First QC.
See that a Telepit is coming, use Disarm to stop the Wand from working that round.
It very useful in certain scenarios and when you are low on mana.