If people can write the dots of classical pieces written orchestras of dozens, jazz pieces by Basie and Ellington and folk played by Bellowhead then I rather suspect that someone somewhere can write the dots for Speed the Bl**dy Plough.

There are a few different kinds of transcription around. Most of the Ellington ones I've seen were a bit skeletal, and you did need to have heard some Ellington to use them. But anyone who wants to play jazz at all will have done. Many trad music scores are in the same spirit: they do tell you what to do, if you have a reasonable familiarity with the idiom. Which isn't all that hard to come by in an age when so much electronic media is available.

Other kinds of transcription are much more informative and much harder to use. That's the kind of thing Bartok did, and which is still very much alive in the jazz world. They use paper as a surrogate recording machine. You need much less familiarity with the original if you're given that much information, but those scores can be VERY hard to read. The amount of rhythmic precision in some of Bartok's "parlando rubato" song transcriptions has sometimes defeated me entirely, and so have some bebop-era jazz solos on paper.

Then there are some in between, like Dunlay and Greenberg's book on Cape Breton music. It's a long way above Mally or O'Neill in precision and historical doumentation but a good bit more user-friendly than Bartok.

There are SOME tunes with individualized histories and performance traditions that aren't readily available in print. What you do near the end of "Lady Leverpark" when playing it for the dance, for example. But the session scene doesn't transmit that sort of thing very effectively. Nor does it generate interestingly varied versions of tunes in the way that the process operated before the session scene came along: the older process used a mix of one-to-one aural transmission and paper, and like the evolution of biological organisms, the fact that it took place along lines of descent that involved very few individuals meant that things diversified fast. Large-group performance blurs and simplifies, it doesn't often create anything strikingly new.

So, I'd say some of the purported benefits of an aurally based session tradition (over a paper-based one) are overrated. Yes, it does get individual participants playing with much better style. No, it doesn't have much going for it as a creative process that develops new music.