User blog comment:Woest/Chapter 382: Alegria/@comment-121.54.44.89-20140428152822/@comment-24066070-20140428172404

The first point is about the potential power-up Tartarus would receive. Perhaps Layla was an very powerful/talented mage, but had to quit due to health problems. If the demonification process would somehow cure her illness or solve whatever health problem she had, then they still have a healthy new recruit who still has all the potential she had while she was human. Therefore, I doubt they would make a judgement based on the health state of a potential recruit, but rather on the potential he or she may have after the demonification.

We don't know how Kyouka's reinforcement works. It may just be useful to give the user an indication of the target's potential (die if it's low, remain unharmed if it's high). After all, Minerva still had to undergo the demonification in order to get the power boost even after being "tested" by Kyouka's reinforcement.

You say they could get the information about the One Magic through torture, should they be interested in the information. Now, which seems the best possible way to go about it? Torture someone of weak health (who may not survive the torture in the first place) and hope you get the info before she kicks the bucket or turn her into a willing slave, who is also strong, intelligent, talented and whatnot for future use? Easy pick if you ask me.