Though this benefits Youki: no records of his existence within the demon king's forces naturally means nobody would be looking for him, but on the other hand it erased his whole previous existence before the moment he moved to the human city. It was a largely meaningless life for him, yes, though he did meet there the few non-human friends he still has. But it was also the time when he met Cecilia for the first time. So, despite being mostly a boring period of his life, it still contains his most important encounter. Not to mention the war against the demons would still be ongoing if Youki hadn't sided with the humans. Not that he would need any other reward for that but the chance to date Cecilia.