Marchisio will be playing a key part at Juve for years to come, so it doesn't matter what imaginary formations and lineups people post he will still be an important player.
De Ceglie can involve into a better player and with Vargas they can own the left side againt any team. Krasic isnt good in defending thats why we neeed a defender as right back and not a Dani Alves. We need a leftwing who can defend and not only attack. Why do you Belive they cant work together?
He is definitely not as good as Melo or Aquilani albeit that they play different roles. Furthermore the balance of Melo and Aquiliani is perfect for a CM pairing, Marchisio would disrupt that balance.
I don't think there is anyway that Marchisio will be taking either spot.