Fuel economy is the only reason to do this...
Jet engines really do not require "warm-up", except in some extreme cold conditions (oil)...
In a 747 we start the "remaining engines" at least 3 minutes before T/O.
With the 747, in my PanAm days at JFK
, taxi on 3 or even 2 engines was frequent. I remember being number... something like 40 or 50 for takeoff. Out of JFK
, late evening departures for Europe. Sometimes the taxi time did exceed 1 hour. Once at JFK
, I also had to return to gate to refuel.
Do not believe that a tail mounted engine does not have much asymmetrical trust. Even on a MD
-80, loss of engine at low speed on the takeoff roll, it is impossible to stay on the runway unless the remaining engine is immediately retarded. There is a VmcG for these airplanes... even if it is lower than airplanes with wing mounted engines, but there is one...