Honestly, if it happened, I wouldn't be mad at the move. I dont think the Niners have much invested in the QBs on the roster anymore. Smith is on a one year deal, Carr is 2 years IIRC, and Davis was a 5th rounder. If they take Clausen, I would be willing to bet that they get rid of either Davis or Carr.
Smith would be the guy until his contract was up since he knows the offense, and then they would make way for Clausen. As far as who they would choose between Davis and Carr, I think Carr gets the axe. I could be wrong, but I dont think his cap hit would be that high, and Davis has been in the system longer.
Obviously, it would not be my first, second, or third choice with the 13th pick, but I would at least understand why the Niners did it. They know they have a really talented surrounding cast, and QB is just as big of a question as the OL.
To be honest, I am more frustrated that the Niners seem to think they can solve the entire offensive line problem in the draft. It often takes linemen just as much time to develop as it does QBs. They have made no moves to improve the line in free agency at all. Not even moves for depth