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You are going to have a hard time pointing to a caster that can shoot effective 400-500 damage shots every 0.6 to 1 seconds all day long at unretaliable range. It's a devastating build against ranged/AP. It doesn't work with every team composition, and it has counters, but it has its place.
Proper AP Kog is misunderstood by the nature of language applied. Stacking AP is inefficient on him. Stacking mana and Mpen, on the other hand, is very efficient. You simply get 250-300 AP as a side effect of building that way, which is why I think people refer to it as "AP Kog."
I agree that attack speed is still very useful on that kind of build. But R can be a primary attack. It's just a spell that you scale with CDR, mpen, and mana capacity rather than AP. It takes significant Magic Resist and HP to shrug off a continuous artillery kog barraging.