Which Haven heroes do you prefer?

That's true, but when there are many Knights (if you actively train them), they become a crucial stack on their own, and Axel reinforces this advantage. Although, it might not be as beneficial as focusing on some other stack.

You see, if there are many Knights, they will justify themselves anyway, and if you also have an enhanced stack of Pikemen, the advantage becomes even greater. It all depends on what you are aiming for in terms of upgrades.