Might be true about too many conditions in one go. I've thought it a little strange that many mages like to pile into one zone when fighting someone who controls disease. That just always bugged me. Without poison gas cloud being dark, it's hard to deter that strategy.
The idea is that, since the necromancer obviously can't consistently cast both, instead of all zombies losing pest slow lumbering and getting bloodthirsty 1 and fast, they just apply poison more effectively. Most poison zombies can't even hinder fleeing creatures without a frenzy.
In light of that, and all zone attacks scaling wildly depending on the number of creatures in a zone, does it still seem like too much? I'll mull over the idea of spreading individual conditions.
Edit: removed a question since I came to a better understanding of echepron's suggestion.