Schlonger,
I guess it is all relative - Virgin has significantly more money than Ansett!!
I guess that the real reason is that the administrator wants to keep the airline as intact as possible - Virgin would not - I imagine - want to pay the costs associated with operating AN aircraft with AN's overheads - ie wages, terminals etc.
I don't think that there is a serious risk that Virgin is uncreditworthy - the administrators would get paid. Boeing and the leasing companies would not be providing them with new aeroplanes if things were so tight.