Range!
I also think that Airbus inted to add range to itīs two engine fleet, for it to be used Europe-Asia nonstop.
And the companies can probably use the same qualification during transit to save money. I fly 330/340 combined and it works perfectly well, its a nice aircraft, best I have flown so far. Though we have some problems with maximum ZFW(ZFM..) due to lots of cargo.