Another use case: upon reading A.Baxter's Porting to 3 talk, I realized,
slightly generalizing from his example, that
print(s.join(map(str,it))) == print(*it,sep=s) -- or would, except that
it currently has to be written non-intuitively as print(sep=s,*it),
which I might not have tried except for knowing about this issue.

Given that many have problems with .join and that most uses are to
produce immediate output not otherwise processed, I think having the
replacement work in the way many would expect would be a win.
So I hope this makes the next beta.