4-3N-3 is a formation that plays through the middle and if his MCs don't perform good he's doomed. There's no wrong use of an MC and as I said - your team probably just doesn't depend on the MCs. If so, then you don't need to worry about that. It's like the GKs that regularly get a rating of 5 or 6 even when they don't concede a goal. If they don't have enough work to do - there's nothing to rate.