If Taysom can't play for an extended period of time then Tanner plays. If Taysom is fine then he redshirts. It at least gives him a chance to play in 2019 if he needs to.
The only thing you lose is the inability to play him a couple of series a game which isn't that big of deal. If we are all confident that Tanner is a NFL talent, then we can be equally confident that he can be an excellent quarterback for us in 2017 without any meaningful game action in 2016.