They cant take a QB for the sake of taking a QB.
With that said, if there is a QB up there they see as a franchise QB, that is the pick.
It is likely the Bills will get 1 or 2 more cracks at the front of the draft. Hypothetically, if they fall in love with him and if Luck doesnt come out and they dont like Locker or Mallett, they could potentially get Luck down the line. Why not draft the BPA who isnt a QB?