I perform an Arcane Missiles right into it from Emilylowes's blog

So even if you don't possess a proc, let's say you're at WoTLK Gold 60x and you don't own a missile barrage proc, you'll be able to cast your Arcane Missiles and it's been dubbed missile barrage, gambling. And the reason for that is because after you have completed the last Arcane Blast, like right here, if I perform an Arcane Missiles right into it there's still a 40 percent chance that the Arcane Blast finishing the cast will give me an opportunity to launch a missile Croc.

On average that means hard casting Arcane Missiles like that will always be more powerful DPS as compared to Arcane Barrage in the case of dumping your stacks, since you're forced to change to a more conservative man rotation. 

This is the reason the reason why you can have instances where specks don't take up Arcane Barrage at all, because you just don't use it for dumping stacks. Instead, you make use of Arcane Barrage for WoTLK Classic Gold doing damage in the course of moving.