another question I have also wondered about, again with operating costs in mind is why don't aircraft at the ramp use 'shore power', why run up hours and fuel running the APU to provide aircon and power when alternate power is available.
My old airline switched to ground power and conditioned air on almost all transits. On the surface, it does look cheaper and cleaner. However, here are a few factors which add to the costs (sorry for the off topic thread creep):
Additional manpower required to install ground power/conditioned air (e.g. on A380s, our airline insisted on installing 4 power leads and numerous conditioned air supplies... very labour intensive).
Medical expenses due to injuries incurred during the fitmentment of such. Overtime costs when replacing injured staff.
Occupational Health and Training course expenses (when the company realises so many people are suffering injuries)
The provision of expensive (sturdy/safe) steps at almost every gate for installing power/air leads (sometimes the aircraft connections can be quite high off the ground).
Aerobridge operating training costs (where power/air is supplied by connections on the aerobridge). On this type, there are interlocks preventing you using power/air connections when the aerobridge is in motion, so arriving flight crews have to start the APU anyway until the Aerobridge has been fully docked with the aircraft.
Delay costs due to engineering manpower being diverted to the removal of power/air when they could be fixing/servicing/certifying the aircraft.
Maintenance may require APU operation anyway (bleed air may be required for engine ground runs, airconditioning pack faults, etc).
Ground power/air isn't always serviceable.
Ground conditioned air isn't always sufficient to cool a large aircraft on hot days.
The switching from external power to APU electrical power is not always smooth and can cause glitches (which can cause delays if the APU is started only 5 minutes before departure).
I've probably missed a few.