i think they signed mullens because they don't want to carry 3 QBs, rostering just mullens instead of mond and mannion frees up a spot for someone else. hanging on to mond because of his draft capital would be a great example of sunk cost fallacy, but NFL teams make those mistakes all the time so i guess its possible.
but it is not a good sign that camp reports indicated mond was struggling, then he lays two eggs in the preseason (except for one nice drive), and then they go and sign another backup QB who has started 17 games in the league. mond is hanging on to a spot on the 53 man by a thread right now IMO. will be interesting to see how it plays out.