The engines are also used to produce bleed air which powers the air conditioning system which in turn pressurizes the aircraft, in addition to other uses.
extremely critical takeoffs the there are performance charts for a "packs off" take off, which in the 400 has the APU providing bleed air for the a/c. The use of the APU for takeoff provides about 1500lbs/thrust/engine.
As far as individual a/c that can use their APUs in flight, are you talking about starting in flight or just use? All the new generation twins can start and operate the APUs inflight. Don't know about the 340, the 400 can operate in flight, but not start. Some of the 747s can depending if the operator has modified the APU inlet door.