Mastery of mutations
"What is it?"
"Your experience."
What experience? My Soul System experience?
"Most Saviors cherish their experience like gold. They hoard it until they have enough to advance to the next level. They don't care if they have to go through tens of missions every month, as long as they earn enough experience, then they'll do anything to get it.
Now, if you tell them that they have to 'waste' their experience on their mutation that only gives them a partial increase in strength, they would look at you as if you were insane. I would know; I've tried and failed to convince hundreds of Saviors to improve their mutation like I have.
If they were given a choice between increasing the mastery of their mutation or increasing their LifeSkill by one stage, then they would most certainly choose the latter. A LifeSkill's jump in power with each stage is very prominent. It makes them feel as if they have become two times stronger than they were before.