Some prognosticators believed that Stockpile may be good enough to replace the infamous Pack Rat in Mono-Black Standard decks after Return to Ravnica block rotated from the format. Unfortunately, there weren’t enough Zombies left in Standard to synergize with it, especially after Jarad was no longer around. Grim Guardian and Gray Merchant of Asphodel were the only two really notable Zombies in the Theros block, and neither of them were something you want to discard for a vanilla 2/2 Zombie that a) comes in tapped and b) gives no devotion.
What about in Modern? Gravecrawler seemed like the perfect thing to synergize with this card. He’s a Zombie that can recast itself from the graveyard for a single Black mana as long as you control another Zombie. You can get really nice card advantage with this combo, as obviously you’re making the Zombie that’s necessary to recast him.
Also, while not a Zombie, Bloodghast can easily come back from the graveyard with a land drop, so discarding him before playing a land can net you a card. But the Stockpile is a bit too slow for the high-powered Eternal format, so outside of casual brews, nothing ever came of a deck built around this engine.
In Commander, this card can be quite useful. Zombie tribal decks are can be quite bonkers in the format. But outside of seeing a fair bit of play in Gisa and Geralf decks, it sees only fringe play in the format. Grimgrin, Corpse-Born and The Locust God play it sometimes. Other decks that have played it on occasion include Mikaeus, the Unhallowed, Ghoulcaller Gisa, and Thraximundar.
A few other Commanders who have made use of the draw engine that don’t care so much about the extra Zombies but can get value out of what’s discarded include Sidisi, Brood Tyrant, Sheoldred, Whispering One, and Alesha, Who Smiles at Death. It is a bit surprising that Jarad, Golgari Lich Lord is not among the top Commanders that utilize Stockpile, according to EDHREC.
Overall, Necromancer’s Stockpile is a solid rare Enchantment that knows what it wants to do. It would’ve been a lot better in Standard during Innistrad block when Zombies were very viable. It does what it does very well, and in the right Commander deck, it’s pretty useful.