Because I think he's more of an asset at 9. PJ wouldn't play if he thought there was chances of reinjury. I think it is in our best interest to have either Latu or Marsh, dependent on the injury situation, raking it for us. You only have to look back at the Parramatta match early in the season, and the come from behind win against Canberra at Ericsson to see that a dynamo like Marsh or Latu is a match winner, especially for a big unit like the Warriors. One quick play of the ball, a tiring defence and either of those two have the ability to snipe into a gap, or at the very least give their forwards a run from 5 metres in front of the Ad line. It doesn't necessarily save you in the halves either, teams will continue to target a Campbell or a Jones if they think a player is hesitant in D.
Monty to me is a natural lock these days with his knee injury slowing him down. I thought his performance was underrated last year, because while he didn't have Marsh/Latu's scintillating pace, he could muscle players off to reach 10 metres from dummy half, to go along with his D. But I think his defensive work rate is best used roaming around playing lock.