deck idea from gatherer:

I find this card as useful as Dark Ritual, especially in my Spirit Reanimator deck. I use it on creatures like Nether Shadow, Nether Traitor and Bloodghast to free up mana for my instants and sorceries, or for other, bigger creatures. But its uses go beyond that, easily making Culling the Weak 1 of the 3 most important cards in this deck.

Example -

Turn 1: Swamp, Dark Ritual, Entomb Iname, Death Aspect, Exhume Iname.
Turn 2: Swamp, Culling the Weak, sacrifice Iname, raise 4 Nether Traitor in addition to the 4 Nether Shadow I raised at upkeep and the 4 Bloodghast I raised when I played the second Swamp.

I then have that extra Black from the second Swamp to Unearth any of the more use-specific Spirit creatures, such as Kataki, War's Wage (if I'm playing against Affinity/Artifacts) or Kira, Great Glass-Spinner (if I'm playing against Control/Removal). I can otherwise save those BlackBlackBlackBlack I got from Culling the Weak to end the game early with Mortal Combat, or similarly win by using those 4x Black and the extra Black I got from the second Swamp I played to raise all my Spirits with Patriarch's Bidding.

Culling the Weak is useful in many other situations. Say my opponent drops a fattie that could really hurt me as soon as it hits the table and I only have 1 Swamp untapped but no Dark Ritual -Culling the Weak and Rend Flesh. It also helps me cast that Angel of Flight Alabaster with just Black and the White I get from my Cavern of Souls. That's BlackWhite for a 4/4 with Flying that returns 1 Spirit card to me every upkeep (and that includes Nameless Inversion). I can then use Nameless Inversion as removal for small creatures or on my Thief of Hope for a small combo engine.

The sac outlet and 4x Black Culling the Weak also helps me cast my Karador, Ghost Chieftain early and when I don't have enough creatures to reduce his cost to BlackGreenWhite.