An expensive board wipe is overkill if you just have one creature to deal with, and a single target kill spell is pointless if you're being harried by lots of little creatures
Delirium has proven to be something people can enable relatively consistently when they want to in the past ( Shadows Over Innistrad ), so with the right support suite, this card can just be Demonic Tutor a decent amount of the time
When I do use them, I always want them to be justified, like putting in snow falling down from the clouds or lightning striking from the sky
Also, dont forget that it cant be targeted by instant spells, so your opponent will have to have either a board wipe or sorcery speed removal spell to stop it.
Since it counts your draw step, your zombies essential start with +2/+2, but you can easily accent that thanks to cards like Distant Melody, Varina, Lich Queen, and Kindred Discovery