Being the type of guy who was always stuck out on the chicken-wing, I was always told to never let anyone get outside you, unless you are taking them into touch. Of course, things are a lot different now with kicks to the corner etc, but it still seems that wingers often come in way off there wing, mainly to help defend against big running centres. The solution is to have even bigger defending centres, aka Big Jamma.
Also, the battle is for the ruck these days, so you want 3 players in a tackle, first to dance, then to wreste to his back, and then they peel off, slowing it down even more. Every fraction of a second allows your defensive line to reset.
Or it could just be this... that comes down to confidence of the coach in his players. He selects the team, he should have faith in his selections.