It has nothing to do with "optimal". It has to do with "living up to expectations". If the player wants a powerful anti-magic juggernaut, Forsaker does not do that, even at lower optimization levels. If the player expects and looks forward to something Forsaker does not do, and you know that, it's worth sharing your insight. I imagine that CMagnum knew what their fellow player was expecting, and knew that it wouldn't work out with Forsaker levels.
I mean, the books are absolutely horrible at telling you what works and what doesn't (even to the point of misrepresenting class features, sometimes). The Forsaker class description, for example, says: "By depending upon his own resources alone, the forsaker becomes stronger, tougher, smarter, and more nimble than any of his companions". It's wrong, but anyone reading up on Forsaker could think "oh boy, this is awesome, and look, the book says it's powerful, too". It's easy to get disappointed after that.