Reason for Sacrifice
A Tier 4 being could easily trump a hundred Tier 3 beings from his/her race. Another thing that led to a qualitative leap in their strength was their possession of an Unranked Skill.
The Parasites, whether Wisdom or Frenzy had six segments, meaning they were capable of six evolutions.
When the Parasite evolves the fourth—with the first being the Induction Ceremony—time, it gains the ability to exist independently. If it chooses so, it can roam the Wilds. The only reason it refrains from this option is that the moment it does so, it would lose all scope for growth. Moreover, it wouldn't be able to use its Skills, not to mention its body paling in comparison to the host it had Nurtured.
So, unless the host died, the Parasite would never resort to living out in the open.