User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170213165134/@comment-1359322-20170215033224

@Pawn, the true purpose of the Reincarnation system is to replenish the Devil forces that was depleted in the Great War, not to replace the origins of the Devils. If you're not a Pure-blooded Devil then you're not a Pure-blooded Devil, your children won't be Pure-blooded Devils too. Why do you think Extinct Clan exist in the 72 Pillars when some of their members are still alive. Because they have blood of other species in them, therefore lost their Pure-blood status and are considered extinct. This is all there is, Reincarnated Devils aren't Pure-blooded Devils. Their children while they may be Devils aren't Pure-blooded Devils too, and since there is no specific term handed to those type of children, we'll classify them as Reincarnated Devils as well, so that there's no confusion.

The purpose of the Promotion system is to motivate the Reincarnated Devils into working for the Underworld. You can't expect them to be motivated to work under anyone or the Underworld if you tell them you'll forever be a servant and Low-class Devil.