Have you noticed that the Patriots, a team that plays the 3-4 the majority of the time, have spent 1st round picks on DL the last few years? Have you also noticed the success this defense has had?
Did you see what Ted Washington did last year when he played NT for the Pats? He didn't get many tackles or sacks but he manhandled the guys in the middle and freed up the LB's to roam free.
NT Casey Hampton does the same thing for the Steelers. He is the cog in the middle.
Bottom line is that if you don't have some good guys up front, the LB's are worthless.