This obviously boosts the worth of Flame Imp and Doomguard, two cards that are already cornerstones of Zoo. It could also be enough summon Pit Lord off the bench, but I'm skeptical as without the Voidcaller, Pit Lord will still hurt entirely too much to play in most situations. Voidcaller should fit easily in the Dark Iron Dwarf slot. Defender of Argus could be subbed out instead, but realistically, it's entirely too good to drop.
What it will likely come down to is a safe approach versus a more risky one. Dark Iron Dwarf is a 4/4 for 4 that is almost never a subpar play, given Zoo's powerful ability to control the board. Voidcaller will be less of a certainty. If the warlock in question has no demons in hand, Voidcaller is statistically underwhelming, being a 3/4 for 4. Moreover, it could actually drop a Voidwalker earlier than intended. Zoo often requires road blocks put up at key points to protect other minions, or avoid blows to the face. Conversely, if a destroyed Voidcaller summons a free Doomguard, that will quite likely end the match. Voidcaller is all about risk and reward, which is fitting considering the warlock set in Hearthstone.