OP...
The amount of aircraft doing SE taxi is dynamic. This changes on a flight by flight basis dependant on a number of factors, predominantly taxi time. Once the APU is up and running and a sufficient cool-down period is completed (3 minutes for us), then the option to conduct SE taxi is there.
I'm yet to see anyone take it up taxiing out and imagine that the only time it would be is when the time to take-off is significant (above 30 minutes). In this case I would also suspect both engines would be started initially, and provided it's a normal start then would proceed to shut one down (and eventually restart) after being notified that the time to take-off would be extensive. I assume the provision is there to accommodate 'unusual' situations or for airlines running out of busier airports (Heathrow?) where taxi times are consistently long.
As for taxiing in, we burn 1.2 tonne/hour at idle (B767, two-engines), so discounting the extra fuel for the APU and the higher power settings for crossing runways or getting onto the bay/during turns, I guess 2 minutes you're looking at 60kg of gas saving per time we do it.
Hope this helps.
