Poison Explosion might have been a useful skill for a Poisonmancer, or any Necromancer for that matter, had it not been in direct competition with Corpse Explosion. Unfortunately, the latter skill is so devastating and effective that it makes no sense to ever use a corpse to release a poison cloud. The result is that this skill is often only used for synergy purposes.
This spell permits the Necromancer to accelerate the decomposition of a corpse to an alarming degree. So rapidly does the corpse putrefy, that the toxic gases, normally accumulated in dead tissue over time, explode forth from the carcass in a cloud of poisonous vapors.
↑Diablo II Manual, page 74. Blizzard Entertainment, 2000.