Invulnerable saves: Mr. Emperor's Champion uses the Iron Halo, so the only guys in the army now with invulnerable saves are the Chaplains. I only take Marshals or my High Marshal in big games that use multiple detachments.
Purity Seals: Purity Seals are good for templars. When a Templar falls back, it goes forward. Say there was a wraithlord there in front of them, or a unit of howling banshees. Would you want your squad to be horribly mutilated? That's why we have purity seals. If there is a wraithlord or something strong there, use purity seals to pick the 3 lowest dice, so you can go 3" forward instead of 12" forward. When there is something weak there, like an imperial guard platoon, pick the three highest dice to plow through them.
Replacing Devastators: Using a command squad with 2 heavy weapons and 2 special weapons can replace devastators. A less expensive alternative is a tactical squad with plasma rifle and cannon.