Reread my prior post. I clearly said that if the life of the symbiont was NOT at stake, joining Ezri would be unethical (because she was not vetted, not trained, and not carefully matched with a compatible symbiont). And, joining any animal with a symbiont just for a giggle would be like trying to create a Frankenstein monster.
Because the symbiont's life WAS at stake, joining Ezri was a judgment call: the life of the slug vs. the life of the young woman whose life they were going to turn into chaos.