Lyman, Chris
In the BOS case, it is SOP to use APU power only for normal turns. There is a limitation on the APU that once it is shut down, it requires 25 minutes cooldown due to shaft bowing before it can be restarted. So, on a 90 minute turn, they start it on taxi-in to the gate, and leave it running for the entire turn. They don't even bother with GPU power unless the aircraft is going to be on the ground for a longer time.