Every weapon already has a set weapon damage, Monks and Ninjas dual wielding would mean their weapon damage is being lowered to compensate for the second slot. You'd now need 2 weapons to have the same DPS you had before, which would cause people to not only whine but hate RNG more, as Dragoon would stay relevant with just the one weapon. Now, if you want only the main hand to be your damage, then the offhand would just be a stat stick, and really not needed