Yes it is,
First of all orb special skill deal 200% WD, blast deal 150% damage. With 20 heroic on orb skill +100% damage, the damage for normal special orb will be 400%, while using discordance, the damage between orb and blast are swapped, mean orb deal 150% damage, with 20 heroic on orb, +100% damage, orb damage now are 300%, discordance effect, each multi attack grand +50% damage on special considering that become primary, with +2 multi attack, it boost orb damage by 100%, mean my orb damage are 600%, don’t forget, its discordance, spamable orb bcoz its my primary now, consume low MP and yet deal higher than normal orbs special. Marvelous.!!
Beside reactor arc deal damage base of orb damage, mean higher orb damage, more higher arc damage become.
And since arc effected by orb damage while orb damage effected by living force. So its very very very worth it to boost orb damage from the very start.
Example
Orb 600% on return +100% with living force the orb damage become 1200%, arc deal +62.5% orb damage per 0.5 sec. Arc also beam up to 5 target, which is gonna trigger arcanist set affix effect +20% damage for each enemy with arcane debuff, 5 target with single orb/arc= +100% damage,
Now count if I just spam orb, lol there gonna be hellish a lot arc beaming and a lot mobs effected there~
+2 multi attack are important and will give very big different in damage. Hope that the explanation are clear and understandable.
Opss very long explanation for only +2 multiattack. Haha