The more bleed demand there is; the more fuel is used, simple as that......
We use one engine and one pack to taxy out. Then no packs for take off.
I've just had an amazing idea....why don't we all charge a sensible fare to the passengers, so that we can run both packs, have our cabins at a comfortable temperature, train our cadets properly, have proper rest, proper recurrent training and decent ground handling, including baggage, etc., etc,.......
The passengers would have good leg room, and good in-flight service, and a decent back-up service if things went wrong.
Sorry about the thread creep, but Just a thought...