The right name for it is MFF (mixed fleet flying) and CCQ (cross crew qualification) and both is very common in the Airbus world and intensively promoted by Airbus Industries. There is hardly any A320/330/340 operator that hasn't done it.
The reason why certain airlines gave it up (like LH) is that fleets are so big that there is no disadvantage of having two different kind of pilot corps. So you do again one short and one long haul group, which improves the flexibility for crew planning.
I'm surprised that QR hasn't had it before ...
Dani