I disagree with that. In reality it removes an option. When an opponent ignites or poisons an area around (and including) your character there's a few options available. Running out of the impacted area should be one of those options, but instead it's the worst move you can make because it inflicts the most damage. Standing perfectly still inside the impacted area is actually a more desirable option, which tactically makes no sense.
Yeah, you can stay, in fire and smoke, and be useless. Or you can run, taking a lot of dmg. Or you can think about some way to remove damaging surface. Or use teleport to get out.
With your proposed change, there would be only one "desirable" option - run. And you could just ignore surfaces when trying to reach enemies. "Go around? Meh, I'd rather take 20 dmg goin through fire, who cares?"