Playing arcane mage almost exclusively in M+ and pvp content I can't grasp why all the raid guides I find don't recommend Arcane Orb at all for cleave/AoE fights (like Mistress and Host.) The ability to generate full arcane charge with a single GCD is insane, especially when you consider how much damag arcane barrage does if you can manage to hit stacked targets.
Paired with the newer legendary shoulders from 7.2.5 arcane orb is actually insane, and yet guides seem to always default to overpowered because it's passive and gives you flat extra damage and mana efficiency.
Arcane orb is extremely fun and extremely good. It's shrugged off as a gimmick only used in PvP but it's actually insane.