The inside linebackers play poorly when they are playing zone. You want them to be playing man or blitzing for them to be effective. On a Blitz they are at their most aggressive. When they are in zone they play soft and don't tackle with aggression.
This is one of the biggest flaws in football games an I even made a thread about it in the Madden forums. The MLB's and OLB's run defense is tied to the coverage. or to say it another way, the coverage you choose determines their run defensive behavior. This is soo wrong it would take days to talk about it.