Before its Commander 2015 reprint, Necromancer's Covenant was included in decks with popular White/Black Commanders such as Teysa, Orzhov Scion, Teysa, Envoy of Ghosts (also in the Call the Spirits deck), Obzedat, Ghost Council and Vish Kal, Blood Arbiter. But in none of those decks has it been a staple. With Karlov loving any lifegain you can muster and Daxos the Returned caring about any Enchantment you cast, this Covenant would see more play.
While the Covenant has a bit of a narrow effect, late in Commander games, you'll usually find a player that has enough creatures in his/her graveyard to have this be worth casting. Plus, if you're already going the token route with Teysa, Orzhov Scion, having some extra Zombie tokens around will certainly help, especially Zombies with lifelink. If you're looking for another Enchantment to round out your 99, this is a good option.