A300Man,
With regards to back up aircraft QR have to small a number of flights that would warrant having an aircraft remain as a back up or to lure in an ACMI company to base aircraft at DOH. The reason why you see aircraft remain for long hours on certain routes is either for slot availability (in case of EURO airports) but the most likely reason is flight connections since 62% of QR's passengers are only connecting and not O&D. Anyways the schedules to DXB have been modified and now the aircraft would spend much less time in DXB than earlier.