What's to stop us gaming the mutation system?

If say, I’m struggling against grenade wielding crabmen, but I find that fighting the melee crabmen is a breeze - what’s to stop me from leaving a few of those melee crabmen alive (be it on the map, or letting them flee) in order for me to increase my chances of being able to selectively fight that melee type of crabmen on future missions?

What about the swarm of mist spawners, and each one could kill all your troopers on the field on next turn?
Or maybe a Hiron mutation, which corpse will destroy your aircraft in the air with 50% possibility… and all other stuff for not being bored?
Or you prefer worm, who will hide into troopers arse and destroy your manufacture, after your trooper will sits on base’s toilet to relax?
No-no-no! Better to have Siren’s scream, wich will erase all current researches, including scannings.
Ah, all of it already existing.