Some people just couldn't have understood the weight of this card. The basic idea is that although C'thun has enough power for a 'starter deck', its battlecry damage alone is not quite enough for a direct win. It can be easily seen that your chance of an easy C'thun win is dampened by enemy minions on board, whose health can be modeled. The model turns out to be that C'thun is still a little behind the competitive frontline.
So the winning strategy offered by this card Doomcaller is essentially a second C'thun shot. Blizzard has thought about it, and thus this card in every template C'thun deck. The reasoning is good, whereas the card also turns out useful. But since some mainstream removals can void the shuffle-back effort, the whold C'thun system is still not robust enough for everything.