He is a fantastic player, as Nonu was for the All Blacks, however he didn't stop improving, in the end he became a great all round centre with kicking one of his strengths.
At the moment, he does not have JJs defensive skills and short kicking game or Slades distribution and kicking ability. JJ is a proven international and Eddie trusts those that have performed for him in the past, hence the recall for JJ and even Clifford.
Marchant is still young but with a huge future ahead of him. The youngsters are starting to catch him up eg Dingwall from the Saints and the young kid Lawrence at Warriors.