Board Thread:Speculation House/@comment-25110963-20160214165742/@comment-26397825-20160214220810

Well, it is an extremely rare gentic quirk, and may not be something that develops in all members of the family. So it's useful, but because of the rarity it can't really do much in terms of large scale extermination of the Grimm. Plus it may leave the user practically comatose, like Ruby was left, so the risks sort of outweigh the benefits.