Poison Nova generally does equivalent or greater (and occasionally a bit less) poison damage per second than Poison Explosion, depending upon the exact roll on the damage. This means that even if you use Poison Explosion, it will almost certainly either (1) have zero effect as the current Poison Nova is superior in damage, or (2) be quickly overridden by the next Poison Nova and have zero effect. In those terms, it would be better not to use the skill at all, unless you are using exclusively Poison Explosion without any Poison Nova at all for the longer duration. That is beyond inefficient, considering the relative abilities of the two skills to actually hit their desired targets.
A maxed Corpse Explosion can kill things not even on your screen, which separates it from a single point Corpse Explosion buffered by however many + skills. The skill deals the exact damage within its own radius, but the damage is not exactly the same when considering the total number of enemies within their respective radii. It is much more reliable than Poison Explosion (static radius of 4? yards) regardless, not to mention it actually does damage.
I'm not trying to say you can't play the way that you want to play. It's perfectly fine to use Poison Explosion/Dagger, Revives, Fire Wall, and so on if that's what fits into your definition of fun. Implying that Poison Explosion is even close to as efficient as simply Poison Nova + Corpse Explosion, however, is beyond laughable.