Keep in mind that the third 'starting' LB is a bit misleading as most teams play in the nickel with 2 LBs on the field far more often than a base 4-3.
I am pretty sure I saw a stat where the Saints were in the nickel for nearly 60% of their snaps last season. If you don't count short yardage and goal line situations, that percentage is going to be significantly higher. One could definitely say that the nickel is our base defense these days