A zombie minion costs 7, less with graveyard. Eternal servant it. Any turn when no creature is gonna feed your graveyard and you really need that 2 extra mana to summon what you want say next deployment, after attacking with it you sac it to altar to give an attack +2 melee +2 piercing and gaining 2 in your graveyard. so you are paying 5 mana of a used and abused creature and a free ready marker action for that timed effect anywhere in the arena like Hand. It ain't broken but it's neat recursion. Especially if he was gonna die soon anyway or you needed to upgrade your servant to the next big thing you summon and needed that mana rebate.
My point was that Necro would love something like
"sacrificial minion", level 4 undead, cheap cost say 7, worse stats than the zombie minion
That is how to abuse the recursive engine!
I would be surprised if eternal servant is not used for recursion shenanigans.