The Poison Spell can only affect Clan Castle troops, which in my opinion is not a good enough reason to spend my dark elixir on it. A great way to make it more useful is to let defenders who are witnessing their base being attacked in real time drop the spell on the attacker's troops. This addition would be great for defenders, and if people find a way to abuse this power then you could limit the number of Poison Spells used in a battle.
