The fact that either Dudek or Kirkland could be injured is enough reason to keep him, if he wants to stay. You quite simply need the best keeper on the bench that you possibly can. Keepers can, and do, get sent off and injured, and you want someone good to come off the bench. The fact that Peggy has been between the sticks at all this season is good enough reason to keep him.
But, if he wants to leave, I think it would be harsh of PT and GH to keep him there.
I think we will have a hard time keeping Kirkland as Dudek is still young in goalkeeping terms (29), and could remain between the sticks for the best part of a decade. No aspiring England keeper can afford to stay number 2 can they ?