I kind of agree and disagree with this post.
I do love the idea of having a strong ninja in my arsenal without having to struggle to collect the seals for it and then praying everytime to be lucky enough to get him.
However if there is ever such an addition (which is doubtful) we all know that all the players will request the strongest and rarest ninjas such as S.Sasuke, S.Itachi, Ay and event ninjas like Hokage Tsunade, Cursed Hidan, and etc.. and by having these strong ninjas the game will become less and less interesting because, now you will have all those players running around Konoha with super-rares. So maybe if they ever do decide to add such a feature (event) in-game it should be limited to only ninjas that are rare enough, but not the rarest.