Balance, the difference in fuel burn @ 5000 feet and 20,000 feet is negligible. @ 60T @ 5000 approx 2160 kh/hr. @ 60T @ 20,000 approx 2040 kg/hr. Just over 100 kg/hr diff. At that time of the morning you just want to get in and land. The guys made an error in the FMC and did an excellent job recovering from the resulting GPWS...end of story. All the training paid off in spades in this particular case. I would hazard a guess this particular crew will never make the same mistake again, and in the process have provided the rest of us with a friendly reminder that it can and will go wrong from time to time.