From a bureaucratic perspective, I think one of GEN Petraeus' roles would be to provide support for GEN McChrystal when the latter has a policy difference with his civilian counterpart (Ambassador Eikenberry) that has to be kicked up to the next level. The CENTCOM Commander is the logical counterpart to SRAP Holbrooke although I sometimes think that it might even require CJCS or SecDef involvement if preparing for bureaucratic combat with Ambassador Holbrooke.