I think there must have been some pretty strong evidence that Khawaja wasn't up to it in those conditions for him to be overlooked after such a strong summer.
While I agree it's a shame that the Marsh experiment (perhaps predictably) failed, I think it should speak volumes that Maxwell came in at 6 before Khawaja had a chance. The very few overs Maxwell bowled should suggest that on batting alone they still saw Maxwell as the stronger option.
Khawaja will be straight back in for the Ashes and hopefully we look to young guys like Lehmann and Cartwright before we consider SMarsh again.
It's a while away but the 15 man squad for the Ashes I'd like to see:
Warner (on thin ice), Renshaw, Smith, Khawaja, Handscomb, Maxwell, Starc, Cummins, Hazlewood, SO'K, Lyon, Sayers, Nevill, Cartwright, and one of Lehmann or Pattinson.
No doubt Wade will be there which I think is a shame.