Er, Wizard does 2450 damage (on average with ElDet), reduced by 20 * def so more usually 2000 or less (there are a lot of things with less than 20 def, but most of the things you want to target with a spell bomb have more, often much more).
So any time it hits 3 or more enemies a t5 or t6 Poison does more damage – 2700 so much more. Basically Poison is better when there’s normally multiple enemies, such as anywhere in Realm including godlands, clearing dungeons, many event and boss battles.
And that’s not considering the other problems of Spells. They are hard to use against moving targets, especially when there’s lag. And they are overkill against anything other than event/dungeon bosses and a very few dungeon minions.