One need only look at 2002 to see what happens when you don't have any depth behind your QB1. Carlyle Holiday was already a terrible fit for the stripped down West Coast Offense that Willingham wanted to run, and the two best passers (Arnaz Battle, Jared Clark) had already been converted into a WR and TE.
After Holiday went down with that blow to the head against BC, our next two QB's on the depth chart were walk-ons (Dillingham, Novakov). Yeah, not exactly a smart situation.
Thankfully, we'll still have three quality players (Carr, Minchey, Herbert).