I sort of agree that he seems too expensive for what he does. 17 mana to protect a conjuration is okayish, but then... he's pretty intimidating when on guard. So what's to stop the other mage from just working round him? If you can take the pain from whatever Cerberus is guarding and use your 17 mana advantage to kill his master, cool. If you can't ignore the conjuration, why not kill it with attack spells and avoid the Cerberus issue that way? If you can do that you've not just taken out the big threat conjuration but also significantly weakened a level 4 creature, potentially gaining tempo from what might otherwise be a purely reactive move.
As a Warlock, if I really want a conjuration safe I might want it on the far side of the arena from the other mage, where he has to overextend himself to hurt it (or, better, can't reach it at all). At that point, Cerberus is a pretty tough creature to justify.
I think that if I cast Cerberus I'd want to be practically guaranteed that he'd get to use that glorious triple counterstrike of his, and where he'd be using it on a target that mattered. I'd want a plan where I'd have him going on guard in the same space as me, and preferably the enemy mage too. And if possible I'd want a backup conjuration in the space with me, so that a quickcast attack spell didn't leave him all deflated in the face of an oncoming Golem. Trouble is, that'd require some sort of new Dark conjuration I haven't seen yet, sort of like a Dark Tanglevine or something equally interesting. Or a Wall of Bone, if that worked (and I suspect it doesn't?).
Here's waiting on the rest of the spoilers then.
