Extracrew is an Iberia's invention far away on time. There are some places nowadays doing the same, transmitted by the spanish origin. For instance Aerolineas. The most curious case is KLM. There are some dutch pilots who have been flying in spain with denimair. They began here to use the xtracrew figure, using and allowing it. Today when they have become KLM pilots they have continued transmitting the idea.
As CRJ900 said it is also a question of give and receive. But the transmitter of the idea through the rest of the companies is the one that have enjoyed the advantages of the xtracrew.
That advantages comes not only being but also allowing.
For that reason I take foreign xcrew often.