I had a thought- poison's main thing it has going is that it makes healers damage enemies. We can't see it going as much, but would poison out damage fire if your party were all damaging things while leaving the healers to kill things with them?
Fire deals a set amount of damage each second for a duration of a set amount of time, and is only hindered when it is stacked with itself or put out.
Poison deals the damage of the healers plus the increased damage of your friends plus the damage that they do due to not dying due to the lessened damage.
At what point does poison become the better option?
The thing is we cant see numerically how much damage poison does.