Answer:
Pros: Advances in Education for scientists in order to expand knowledge on the human race... and if used for war, cheapest and most efficient way of winning a fight
Cons: Unethical to kill that many people with gas and you have the risk of killing your own troops if not used properly. And why use biological warfare to wipe out a country if you wont be able to visit that country after the attack to steal resources and land because of the chemicals in the air?