For what my opinion is worth, I agree with Grégory's suggestion because the ',' part of ','.join(...) is almost as unintuitive as the problems Raymond's suggestions are trying to fix.

I was going to suggest a builtin to work on both str and bytes, like join(sep=None, strtype=str, *strings) but that interface looks pretty bad...

I think joinstr/joinbytes according to Grégory's suggestion (perhaps as classmethods of str/bytes?) would make the most sense.