Linebackers are an important element, as they eliminate holes in or outside of the defensive line. That is for both running and passing plays. They force an opposing offense to actually beat either the defensive line or the secondary.
Having a strong set of running linebackers, as Dallas now has with Rolando McClain, Justin Durant, and Carter, means that tight ends and running backs out in routes have a much tougher route.
The safeties for Dallas doe a good job in filling into the box on running plays, so a team really has to be the unit in the passing game.
Dallas corners are strong and very active, and the safeties have improved in over the top protections. They will probably get beaten on a long play or two before the season is over...but the linebackers are the glue between the defensive line and the secondary. At present, teams are having a rough time right down the middle of the field, and that is what good defenses do.
It takes an entire team, to include the defensive front. For Dallas, that is getting continually better, but at least it is filling on running plays and getting pressure on opposing quarterbacks.
If the Cowboys are able to retain Sean Lee, Rolando McClain, Justin Durant, Bruce Carter, Kyle Wilber, and Anthony Hitchens, there can be a very strong/stable unit to start with. That would allow more draft resource to spread around other positions on the defensive side of the ball.