Emergency catcher isn't the issue. I can't remember a time they actually used the emergency catcher (when it was the likes of Bruntlett, Werth, Tomas Perez. Coste doesn't count). This is about wanting an actual catcher in the bullpen and working with the pitchers and other catchers on a daily basis. And about having an insurance policy if Knapp seems overmatched or Rupp gets injured.
The issue, possibly, is that they may not be able to stash the veteran catcher in AAA. Both players could have opt-outs.
It will work itself out (as we always say in spring training). But if it happens, for sure, it means DH and 1B and PH opportunities for Knapp at the expense of catching, while the third-string guy starts once a week and enters some games late if they've PH for Rupp (or need a double-switch. Though Knapp could also do that).
All else being equal I think it might be what they want, they just haven't shown that bulletproof confidence in Rupp or Knapp (actions speak louder than words). And they are probably imagining a future where Knapp is a swingman type player for several years. But if Stassi, or even one of the middle IF guys, has a great spring training, the spot could still easily go to one of them.