Well, obviously as a base for the game and the story. The feeling of NTR is amplified a lot more if you had a "healthy" relationship with your priestess before Yarizo appears. The other option would have been that Yarizo steals all your girls from the beginning, which would be a lot more boring in my opinion. And as I said, I dont think that there will be a lot of extra content for vanilla, I didnt say that where will be no vanilla at all.
It's the same with the Sphilia's Familiar game which was mentioned earlier in this thread. The NTR Route at the end of the game is so good, because you had a lot of "bonding" with your girls earlier in the story and you built some sort of connection. If the cuckolding happens too quickly, then it's just lame imo.