We don't even have a QB on the Practice Squad, do we? I'm really surprised by that!
We need Moore; he knows the offense, and while Rush has outperformed him, we haven't see Rush vs. a game day, more complicated D-scheme with lots of disguise and blitz. Can he make the calls?
It's my guess that if Dak were out for a few plays or a quarter, they'd be more comfortable with a veteran presense. If Dak went down for the season, it's pretty much over, so they'd go with Rush to develop and see what he's really got. But if you have big time aspirations for 2017, you just can't have a backup undrafted rookie QB as your only option. You may even need Moore to run the scout team offense, since his knowledge of systems has to be better than Rush's. Would you want your undrafted rookie backup preparing for each week's defense and also trying to run the scout team offense?
We had to bring in a veteran. I guess they think Moore is the most comfortable option, for Dak and the staff.