I dont have Bengalese, but when reading different articles it seems like the Bengalese can pass a disease onto gouldians, so you have to make sure you have "clean" birds before starting the fostering. Its something to do with the Gouldians being from a dry climate and the Bengalese from a damp climate.
There seems to be a lot organization that has to be done prior to the fostering, getting both species in sync.
With my limited space, having yet another species to set up and keep in sync with the gouldians on the off chance they might be needed is too much for me. It does seem a popular thing though especially in the USA.
I choose not to foster, but I will try everything in my power to help a chick that needs it, even if that meant trying to put it in another nest if all else fails.
I feel better just letting the gouldians do their thing naturally.

It seems to me that over a long time you might just end up rearing a species of bird that cant raise its own young or because the eggs or young keep disappearing the birds just dont bother anymore.