Sminky: This so-called 'process' was an historical accident, born of a time when oral transmission (along with its attendant risk of 'Chinese Whispers Syndrome') was the only form of song distribution.

I kind of agree with that, although I think 'accident' is a bit dismissive; it's a bit like saying the use of horse-drawn ploughs was a historical accident based on the failure to invent tractors. I'd rather call it a historical reality - and one that existed for longer than the conditions that we're used to. But if you're saying there wasn't anything magical or intrinsically valuable about the 'process', I tend to agree. It's just how things were - and the songs that have come out the other side of that process just are those songs.

So how come it has now become a 'rule' to be applied to modern compositions, for which a myriad of alternative transmission streams is available?

It's not a 'rule', it's a description. There are songs that have been through the process, and there are songs that haven't been through the process. Mostly I prefer hearing and singing the first kind.