Well we don't usually play with one, so we're our own yardstick to measure by, and in the big games last season we were found wanting defensively. Our fullbacks always play high so I feel we need someone who can manage that in transition i.e. they spot the danger and get there. That requires intelligent reading of the game and positioning. Arteta has sufficed to some degree, and has been a great club servant, but I think we need an extra yard of pace and an extra ounce of strength than what he can provide.
Edit: was responding to valeo's post